Mexico city train derailment kills at least 20, injures dozens more

At least 20 people have been killed and close to 70 people were injured after an elevated train derailed in Mexico City when a concrete overpass it was crossing collapsed Monday night.

Security camera footage broadcast by local media showed the carriages plunging towards the ground in the south of the city.

"So far we have 20 people who unfortunately lost their lives," Mexico City Mayor Claudia Sheinbaum told reporters at the scene.

The city's civil defence department said on Twitter that around 70 others were injured in the dramatic accident.

Videos posted online show two train cars crashing into one another on the ground, landing at a V-shaped angle. 

Social media footage showed panic at the scene of that accident in the city centre as lights went out, the ventilation system was interrupted and the station filled with smoke. 

Dozens of emergency workers were seen 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.

A car was trapped under the rubble, but it was unknown if there were people inside.

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

Monday's accident comes just over a year after two subway trains collided in Mexico City, leaving one dead and around 40 injured as panicked passengers escaped through dense smoke.

In another incident in January of this year, one person died and 29 suffered smoke inhalation injuries in a fire in the metro's control centre.

(With inputs from agencies)