An elevated metro line collapsed in the Mexican capital on Monday, leaving at least 23 people dead and dozens injured as a train came plunging down, authorities said. 

Carriages were seen hanging from the overpass in a tangle of twisted cables with the ends pointing towards the ground in a V-shape.

"Unfortunately there are 23 deceased," including minors, Mexico City Mayor Claudia Sheinbaum told reporters at the scene in the south of the capital.

Around 65 others received hospital treatment following the dramatic accident, one of the worst to strike the Mexico City metro since it was inaugurated in 1969.

Anxious relatives gathered at the site awaiting news of those believed to have been aboard the train.

Dozens of emergency workers were seen trying to rescue victims from the carriages.

The work had to be suspended because of fears the wreckage was too unstable, but it later resumed with the help of a crane.

A car was trapped under the rubble, but a person inside managed to get out alive, she said.

Sheinbaum promised a structural examination of the affected metro line, which will remain closed, and a full investigation into the causes of the accident.

"We will report the whole truth. Our support to all victims," Sheinbaum tweeted.

The Mexico City subway has 12 lines and carries millions of passengers each day.