At least one person was killed, and 51 residents were unaccounted for.

A major search-and-rescue effort was underway on Thursday morning after part of a high-rise residential building in Surfside, Fla., just north of Miami Beach, collapsed into a pile of rubble. At least one person was killed, the authorities said, and many more fatalities were feared.

“We are bracing for some bad news, just given the destruction that we’re seeing,” Gov. Ron DeSantis of Florida told reporters.

Commissioner Sally Heyman of Miami-Dade County said county officials informed her that as of 10 a.m., 51 people who own units in the building had not been accounted for. That did not mean they were missing, she said, just that the authorities had not been able to reach them. She added that not all of the units may have been occupied by full-time residents.

Raysa Rodriguez, 59, who lives in the part of the building that remained standing, said she was awakened by what she thought was an earthquake, then escaped down an emergency stairwell and off a second-floor balcony onto a rescue ladder.

“I lost a lot of friends,” said Ms. Rodriguez, who has owned a unit in the building since 2003. “They are not going to be able to find those people.”

Daniella Levine Cava, the mayor of Miami-Dade County, said that about half of the 136 units in the 12-story tower had collapsed. “We’re going to do everything we can possibly do to identify and rescue those who have been trapped in the rubble,” the mayor said at a news conference Thursday morning.

About 35 people were rescued from the building, and two were pulled from the rubble, Ray Jadallah, a Miami-Dade Fire Rescue assistant fire chief, said at the news conference. Chief Jadallah said the northeast corridor of the building had collapsed around 1:30 a.m.

Charles W. Burkett, the mayor of Surfside, told the “Today” show on NBC that dogs had been searching for people trapped under the rubble since 2 a.m.

“Just tragically, there haven’t been any hits from the dogs, and that’s a great disappointment,” he said. “Apparently, when the building came down, it pancaked. So there’s just not a lot of voids that they’re finding or seeing from the outside.”

Paraguay’s foreign ministry said in a statement on Twitter that at least six Paraguayans were unaccounted for following the building’s collapse. The Paraguayan newspaper ABC reported that one of those who is missing is Sophia López Moreira, a sister of the country’s first lady, Silvana López Moreira. The newspaper said the family owns a unit in the building.

Lisandro Sabanés, a spokesman for Argentina’s foreign ministry, said at least four Argentine citizens who were in the building are unaccounted for. Three of them were members of a household, he said.

It was unclear what caused the collapsed part of the building to fall. Mr. Burkett said it was also unclear how stable the rest of the building was. He said 15 families were being relocated to hotels. “We don’t know if the rest of that build is going to come down.”

The beachside building, at 8777 Collins Avenue, is called Champlain Towers South and was built in 1981, according to county property records.

Images from WPLG Local 10, a Miami TV station, showed firefighters pulling a boy from the rubble amid piles of debris and rebar. They also showed crews rescuing at least one person from a balcony on a lower floor of the part of the building that was still standing.

“It’s stunning to see in person,” a witness, Mary Parker, told the TV station.

Emergency personnel work at the site of a partially collapsed building in Miami Beach, Fla., on June 24.Credit...Miami-Dade Fire Rescue, via Reuters

Raysa Rodriguez, 59, was asleep in her apartment at Champlain Towers South when the building began to shake in the early hours of Thursday morning — as if there had been an earthquake. On the balcony, she saw a plume of white smoke. “When I opened the door, I’m like, ‘There’s no more building,’” she said.

Her unit, 907, is on the west side of the building, which stayed intact. But a few doors down, from unit 904 and onward on the building’s north side, everything was flattened, she said. She ran inside to put on shorts and shoes and to grab her purse.

Along with some neighbors, including an older neighbor with a walker and a child, Ms. Rodriguez used the emergency stairwell, which was dark and littered with debris, to get to the second floor. A neighbor on that floor had left her apartment door open, and Ms. Rodriguez and the others ran in and onto the balcony, where rescuers were able to reach them with a ladder.

“I lost a lot of friends,” Ms. Rodriguez, who has owned a unit in the building for nearly 20 years, said. “They are not going to be able to find those people.”

Another resident, Barry Cohen, a 63-year-old lawyer, said that he and his wife were asleep when he heard something that sounded like a loud thunderclap. “But it never stopped — it lasted for almost a minute,” he said. They went out onto their balcony of their third-floor apartment and saw rubble and plumes of smoke. When they tried to exit through their balcony, the staircase was blocked by rubble.

“Everything was just decimated,” said Mr. Cohen, a former Surfside vice mayor. “It looked like it had been hit by a missile.” They went down to the garage, but were again blocked from leaving. Pipes had broken and there was so much water that it went up to his chin. They raced back to their apartment and yelled to rescuers, “get us out of here!” he said. “We were just freaking out.”

He and his wife were eventually rescued by fire fighters using a ladder to reach them.

He said that there had been constant construction around the building since he moved there in 2018. “Everyone was constantly being shaken,” he said.

Residents said that a mix of people lived there, including retirees and affluent professionals with young families. The building is three blocks down from the Four Seasons. Ivanka and Jared Trump are leasing a condominium a few buildings away, according to a report in The Wall Street Journal.

Sergio Barth, left, with his brother Luis Barth.Credit...Sergio Barth

More than a hundred people gathered at the Surfside Community Center on Thursday morning, awaiting information on their loved ones who lived in Champlain Towers South.

Among them was Sergio Barth of Miami, whose brother Luis Barth, 51, was visiting town from Colombia with his wife and daughter and staying at a friend’s apartment in the building, on the collapsed side.

Sergio Barth tried all morning to reach his brother by phone. No response.

“We don’t know anything about him or his family,” Mr. Barth said as he clutched a cup of coffee. “Just keep your fingers crossed.”

People who were evacuated from a hotel next to the collapsed building also gathered at the center, including Aaron Miles and Abigail Crosby of Charlottesville, Va., both 20. They said they were woken by fire alarms and a commotion.

“It sounded like a hurricane,” Mr. Miles said. He heard debris hitting their hotel room windows as the building came down.

They were staying on the second floor of the hotel and were evacuated because of fears the hotel would also collapse. When they got downstairs, people were screaming, trying to find their loved ones in the collapsed building.

“We gave everybody hugs, and kept telling them it would be all right,” Mr. Miles said.

— Neil Reisner

Rescue dogs outside the collapsed building.Credit...David Santiago/Miami Herald, via Associated Press

Relatives of Paraguay’s first lady and an Argentine couple with a 6-year-old daughter were among the people missing in the rubble of the residential tower in Surfside, Fla., according to South American officials, news reports and relatives.

Paraguay’s foreign minister, Euclides Acevedo, said in a radio interview that among the Paraguayans who are unaccounted for are relatives of President Mario Abdo Benítez’s family. He identified them as Luis Pettengill, a cattle rancher, and his wife, Sophia López Moreira — a sister of the first lady, Silvana López Moreira. An employee of the family and the couple’s three children are also missing, he said.

Mr. Acevedo said Paraguay’s consulate in Miami had been searching for information about the family. “Our consul and his team are making the rounds at hospitals in Miami,” he said.

Lisandro Sabanés, a spokesman for Argentina’s foreign ministry, said Thursday morning that at least 10 Argentines who were believed to have been in the building are unaccounted for. The Argentine newspaper La Nación has identified a few of them. They include Andrés Galfrascoli, his husband, Fabián Núñez, and their daughter, Sofía, who is 6.

La Nación reported that Dr. Galfrascoli is a plastic surgeon who has practices in Argentina and South Florida. Mr. Nuñez is a theater director, the newspaper reported.

A Chilean man, Claudio Bonnefoy, is also among those missing, according to his daughter, Pascale Bonnefoy. Ms. Bonnefoy, a Chilean journalist who writes for The New York Times, said her father, who is 85, lives in an apartment on the side of the building that collapsed. He lives with his wife, María Obias-Bonnefoy.

“Their apartment is in the rubble,” Ms. Bonnefoy said.

People gather at Surfside Community Center, which is acting as a reunification center, as they wait for information.Credit...Scott McIntyre for The New York Times

People seeking information about family members who are unaccounted for after the collapse of an oceanfront condominium in Surfside, Fla., can use the Miami-Dade Fire Rescue hotline and a reunification center a few blocks north of the site of the building.

To report to officials a person who is either missing or who has been found, family members are asked to call 305-614-1819 or go to miamidade.gov/emergency.

“Chaplains and victim advocates are on site, ready to support the survivors and family members who will be in need of resources,” said Mayor Daniella Levine Cava of Miami-Dade County at a news conference Thursday morning. “Our social service agencies, as well, are coming in — they are going to be here to assist in the hours and the days ahead.”

The American Red Cross is providing food and mental health support at the family reception center and helping residents find a safe place to stay.

“We appreciate the outpouring of support and community members reaching out to the Red Cross to offer their help during this challenging moment,” a Red Cross spokeswoman said in an emailed statement. “At this time, we have all the resources and assistance we need.”

A family reunification center has been set up at the Surfside Community Center at 9301 Collins Ave.

— Sophie Kasakove

Police stand guard near the partially collapsed building early Thursday in Surfside, Fla.Credit...Chandan Khanna/Agence France-Presse — Getty Images

Several witnesses described the scene of the collapse on Thursday.

Nicholas Balboa, 31, said he was outside at about 1:30 a.m. when he heard a rumble. He thought it was thunder at first — and then felt the ground shake.

“I went up the street and saw the dust cloud going through the corridor of buildings,” said Mr. Balboa, who lives in Phoenix but was visiting his father in Surfside. He raced toward the sound and saw the partially collapsed building.

Emergency crews had started to arrive. Mr. Balboa walked toward the site to get a better look. That’s when he heard a boy call out for help.

“We could see his arms sticking out and his fingers wiggling,” Mr. Balboa said. “He was just saying, ‘Please don’t leave me. Please don’t leave me.’” Mr. Balboa and another bystander climbed through the rubble and tried getting closer.

The boy said his mother had been with him, but Mr. Balboa heard no other voices. Using the flashlight on his cellphone, Mr. Balboa flagged a police officer, who rushed toward them with a rescue crew.

The crew dug through debris and used a saw to get the boy out, Mr. Balboa said. “It was surreal. I can’t believe that a building that’s made out of concrete and supposed to stand up to hurricanes and weather just one night decided to collapse,” he said. “To be completely honest, the comparison, the stark image that I had in my mind, was 9/11, just seeing all that debris and rubble.”

Fiorella Terenzi, an associate professor at Florida International University who lives in a neighboring building, Champlain Towers East, said she was woken up by a loud noise.

The sound “was like a big thump all of a sudden,” she said. At first she thought it was thunder but then started to hear sirens. When she left the building, dust was everywhere.

“I could see that half of the building of the Champlain Towers South was collapsed like a sandwich,” said Ms. Terenzi, 59, who has lived in the east tower since 2000. “It really was a shocking view.” Ms. Terenzi said she had seen heavy equipment on the roof of the south tower for the past two weeks.

Lizie Brito, 41, said she was on her balcony at a nearby building when she saw a swirling white cloud of dust come toward her. At first, she said, she thought it was a tornado. Then she heard an explosion and screams as people ran from the next-door hotel. She couldn’t sleep after that.

“Imagine thinking about the people that were struggling, dying there, and then thinking that maybe this could happen to us in our building,” Ms. Brito said.

Allen Rosenbach, 62, one of Ms. Brito’s neighbors, came to the beach Thursday to see the rubble. He received a text from his wife with names of people who were still missing, some of whom they knew. One of his son’s classmates, he said, lived on the side of the building that collapsed.

Hours after the building’s collapse on Thursday, photos and videos captured by witnesses and surveillance cameras were posted on social media:

Mayor Charles W. Burkett of Surfside, Fla., at a news conference on Thursday about the partially collapsed building.Credit...Cristobal Herrera-Ulashkevich/EPA, via Shutterstock

Public officials in Florida expressed shock at the news of the condominium collapse in Surfside on Thursday morning and offered prayers for the residents of the building and for emergency workers on the scene.

“Horrible images emerging the partial collapse of a condo building in #Surfside #Florida last night,” tweeted Senator Marco Rubio.

Representative Charlie Crist shared his prayers for the “residents of Champlain Towers, their families, and the first responders at this tragic scene who are saving lives,” in a tweet.

Mayor Daniella Levine Cava of Miami-Dade County tweeted that she had spoken with President Biden about the collapse: “He offered the full support of the federal gov. to help our community during this difficult time. We continue to work with local, state, & federal agencies as we respond to this tragedy and do everything we can to support the impacted families.”

Mayor Charles W. Burkett of Surfside, the town in Miami-Dade County where the building is located, spoke to reporters Thursday morning: “Nothing like this has happened to us,” he said. “And I’ve lived here my whole life.”

Representative Maria Elvira Salazar tweeted that the incident was “devastating” and that her “prayers are with all of the families & our brave @MiamiBeachPD & @MiamiDadeFire who are fighting around the clock to save lives,” sharing the same message in Spanish.

Representative Debbie Wasserman Schultz said, “While details are still emerging, we pray that the casualties are limited and first responders are safe as they address this horrific collapse.”

Local county commissioners shared their prayers for the victims, too. The chairman of the Miami-Dade Board of County Commissioners, Jose “Pepe” Diaz, tweeted from the site of the building collapse: “My deepest prayers are with the residents and their families on this tragic morning. Thank you to all the brave first responders who quickly responded to help the victims.”

— Sophie Kasakove