MEXICO CITY — A metro overpass collapsed late Monday in Mexico City, sending the cars of a train packed with passengers plunging to the ground and killing at least 23 people, including children, the city’s mayor said.

Emergency workers scrambled to a scene where tilted train cars lay amid tangled wires and twisted metal, pulling dozens of people from the wreckage and transporting more than 70 people to hospitals with injuries.

Dramatic video of the incident showed the overpass suddenly collapsing in a shower of sparks, sending up a cloud of debris as one of the train cars smashed into a vehicle on the road below.

In the chaotic aftermath, desperate relatives flocked to the scene for news of their loved ones, while others scoured city hospitals in hopes of finding their family members.

“I’m looking for my son,” the mother of a 13-year-old boy told a local television station through tears. “He was on the subway — I can’t find him.”

Many families were directed to the Belisario Dominguez Hospital, about a quarter of a mile away, where they waited for updates on the victims.

Floodlights illuminated the collapsed bridge as search and rescue teams tried to find survivors in the rubble. Ambulances, firefighters, the military and Mexico’s forensic department came and went from the scene while a helicopter hovered overhead as dawn broke, surveying the

By sunrise on Tuesday, the search for survivors had largely turned into a recovery operation, with four of the victims’ bodies still trapped in the wreckage, according to government officials.

“It is sad news,” said President Andrés Manuel López Obrador. “We send our condolences to the families of the victims of this accident.”

The crash occurred at 10:22 p.m. on one of the city’s newest stretches of track, Line 12, which was inaugurated in 2012. Local residents had expressed concern about the structural integrity of the overpass, including cracks in the concrete, after a powerful earthquake devastated parts of the city in September 2017.

Hernando Manon, 42, who lives in the area, was on his way home from work when he saw sparks light up the night sky and the train come crashing down.

He said that he was one of many people who had noticed cracks in the concrete after the earthquake. Although the cracks appeared to have been removed, he said, he did not know the extent of any repairs.

Local officials did not address specific concerns in the immediate aftermath of Monday’s accident, but Mayor Claudia Sheinbaum said that maintenance was carried out on the train line every day. The system as a whole has been plagued by problems in recent years.

“At this moment, we can’t speculate about what happened,” Ms. Sheinbaum told reporters early Tuesday. “There has to be a deep investigation, and whoever is responsible has to be held responsible.”

In the meantime, she said, Line 12 would remain closed as the authorities investigate the cause of the accident. Mexico City Metro advised people to avoid the area.

Ms. Sheinbaum did not rule out the possibility of further incidents on the damaged train line, and said that the avenue that runs below the line would be shut down during an investigation.

Marcelo Ebrard, Mexico’s foreign minister, called the crash a “terrible tragedy” in a Twitter post late Monday. “Of course, the causes should be investigated and the responsibilities for it defined.”

Mr. Ebard was mayor of Mexico City from 2006 to 2012, when the new line was constructed, and accusations of poor construction and planning emerged almost as soon as it was opened. The line was closed briefly in 2013 for repairs.

Hundreds of police officers and firefighters cordoned off the scene on Tuesday morning as relatives and friends of people believed to have been on the train gathered outside the security perimeter to seek information.

Ms. Sheinbaum said that at least one person trapped in a car in the mountain of rubble had been pulled out alive and taken to the hospital.