Mexico's President Andres Manuel Lopez Obrador on Tuesday promised an in-depth investigation to find those responsible for the deaths of at least 24 people in the collapse of an elevated metro train line with a history of problems. 

Dozens were injured in the accident, one of the worst ever to strike the Mexico City metro, raising questions about construction and maintenance standards on a network used by millions every day.

The authorities were working to identify the dead, which included children, Mexico City Mayor Claudia Sheinbaum told reporters at the scene in the south of the capital. 

One person trapped in their car underneath the wreckage had been rescued alive and was taken to hospital. Seven of the people transported to hospital were in a "grave condition" and undergoing surgery, she said.

A video on local channel Milenio TV showed the overpass collapsing on top of cars plying on the road below near Olivos station in the southeast of the city at around 10:30 p.m. local time.

It appeared a girder had given way on the overpass but the cause was being investigated, the mayor said. 

Dozens of emergency workers were trying to rescue victims from the carriages. But the work later had to be suspended because of fears the wreckage was too unstable. 

"For now, the rescue has been suspended because the train is very weak. A crane is coming to continue the work," Sheinbaum said. 

Part of the train was hanging from the tracks in a tangle of twisted cables with the front ends of two carriages pointing toward the ground in a V-shape. A car got trapped under the rubble.

(With input from Reuters, AFP)

(Cover: Rescue work underway at the accident site near Olivos station in Mexico City, Mexico, May 3, 2021. /Reuters)