Mayor Claudia Sheinbaum said 70 people were injured, with 34 hospitalised, and people could still trapped inside the train, which was split in two and appeared partially suspended.
Mexico City's civil protection agency tweeted that nearly 70 people were injured in the accident on Monday night (local time) in the south of the capital and were being taken to nearby hospitals.
There were still people trapped inside the train, though "we don't know if they are alive", said Sheinbaum.
Video showed a auto trapped under rubble, with dozens of rescuers searching through wreckage from the collapsed structure.
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News reported that the Defense Department has temporarily provided B-52H Stratofortress aircraft to U.S. Navy carrier strike group was also deployed in the region for support.
Other videos on television stations and social media showed emergency medical crews and firefighters combing through the wreckage looking for survivors.
A train vehicle was shown partially in the air following the derailment.
The line was built when Mexican foreign minister Marcelo Ebrard was mayor of Mexico City.
Its construction was plagued by complaints and accusations of irregularities.
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Almost 100 first responders came to the scene, with eight fire engines, 10 medics, and various other officials. Romero said there were life preservers on board, but it was unclear whether any passengers were wearing them.
Ebrard wrote in his Twitter account, "What happened today on the Metro is a awful tragedy". "My solidarity with the victims and their families", Ebrard said on Twitter.
"Of course the causes should be investigated and those responsible should be identified", he wrote.
An overpass for a metro partially collapsed inn Mexico City, killing at least 15 people. "I repeat that I am entirely at the disposition of authorities to contribute in whatever way is necessary".
"Unfortunately there are dead and injured people".
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This is a developing story.