Mexico City, May 4 (IANS): At least 20 people were killed after a bridge on an elevated section of the Mexico City underground system partially collapsed as a train was passing over it, Mayor Claudia Sheinbaum said.

At least 49 people were injured, some of them seriously, Sheinbaum told journalists at the scene of the accident on Monday night.

At least seven people were being operated on in hospitals and children were among the dead, Sheinbaum said.

Several carriages of a line 12 metro train plunged several metres and collided into each other, videos circulating on social media showed, reports dpa news agency.

Initial findings showed the accident occurred when part of the bridge gave way, Sheinbaum said.

Traffic had been flowing under the collapsed section of the bridge at Olivos station on the south-eastern edge of the Mexican capital at the time of the accident.

Footage showed rescuers using a ladder to pull passengers out of train carriages as they were hanging from the bridge.

Because of the train's unstable position, rescue efforts had to be suspended, Sheinbaum said.

Residents had reportedly complained about damage to pillars along the route of metro line 12 years ago.

There had also been allegations of corruption in the construction of the line.

In 2014 not long after the inauguration of line 12, service was interrupted for months for repairs.