An elevated section of the Mexico City metro has collapsed and sent a subway carriage plunging towards a busy road, killing at least 23 people and injuring at least 79, city officials said.

Rescuers found four bodies trapped inside a rail carriage dangling from the overpass but were unable to remove them. It was not clear if they were included among the 23. A crane was brought in to lower the carriage down.

“We don’t know if they are alive,” city mayor Claudia Sheinbaum said of the people possibly trapped inside the carriage following one of the deadliest accidents to hit the subway system, which is among the busiest in the world.

Earlier she said someone had been pulled alive from a car trapped on the road below. She said 49 of the injured were in hospital, and seven were in serious condition and undergoing surgery.

“There are unfortunately children among the dead,” Ms Sheinbaum added.

Mexico Metro Collapse
A section of Line 12 of the subway collapsed on Monday night (Marco Ugarte/AP)

The overpass was about 16ft above the road in the borough of Tlahuac, but the train ran above a concrete median strip, which apparently lessened the casualties among motorists on the road below on Monday night.

“A support beam gave way” as the train passed over it, Ms Sheinbaum said.

The Mexico City Metro has had at least two serious accidents since its inauguration half a century ago. In March last year, a collision between two trains at Tacubaya left one passenger dead and injured 41 people.

In 2015, a train that did not stop on time crashed into another at Oceania, injuring 12.

Mexico Metro Collapse
Police bar access to the scene (Eduardo Castillo/AP)

Hundreds of police and firefighters cordoned off the scene on Tuesday as friends and relatives of people believed to be on the train gathered outside the security perimeter.

Adrian Loa Martinez, 46, said his mother had called to tell him his half-brother and sister-in-law were driving when the overpass collapsed and the beam fell on to their car.

He said his sister-in-law was rescued and taken to hospital, but his half-brother Jose Juan Galindo was crushed and is feared dead. “He is down there now,” he told journalists pointing towards the site.

Gisela Rioja Castro, 43, was looking for her husband, 42-year-old Miguel Angel Espinoza.

She said her husband always took that train after finishing work at a store, but he never arrived home and had stopped answering his phone.

“Nobody knows anything,” she said.

Mexico Metro Collapse
Rescues move a victim from a fallen carriage (Jose Ruiz/AP)

The collapse occurred on the newest of the Mexico City subway’s routes, Line 12, which stretches far into the city’s south side. It runs underground through central areas of the city of nine million, but then runs on elevated concrete structures on the outskirts.

The collapse could represent a major blow for foreign relations secretary Marcelo Ebrard, who was Mexico City’s mayor from 2006 to 2012, when Line 12 was built.

Allegations about poor design and construction on the line emerged soon after he left office as mayor. The line had to be partly closed in 2013 so tracks could be repaired.

He wrote on Twitter: “What happened today on the Metro is a terrible tragedy.

“Of course, the causes should be investigated and those responsible should be identified. I repeat that I am entirely at the disposition of authorities to contribute in whatever way is necessary.”