An elevated metro line collapsed in the Mexican capital on Monday, leaving at least 20 people dead and dozens injured as a train came crashing 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 20 people dead so far," including minors, Mexico City Mayor Claudia Sheinbaum told reporters at the scene in the south of the capital.

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The city's civil defense department said on Twitter that around 70 others were injured in the dramatic accident, one of the worst to strike the Mexico City metro since it was inaugurated in 1969.

Security camera footage broadcast by local media showed the carriages plunging towards the ground.

Dozens of emergency workers were seen trying to rescue victims from the carriages. But the work later had to be paused 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 a person inside the viaduct managed to get out alive, she said.

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"Suddenly I saw that the structure was shaking," an unidentified witness told the Mexican television network Televisa. "When the dust cleared we ran... to see if we could help. There were no screams. I don't know if they were in shock," she added.

Medics were seen taking the injured away on stretchers.

The casualties were rushed to different hospitals in the city, Sheinbaum said.

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The accident occurred when a section of the elevated tracks collapsed at the Olivos station at around 10:00 pm (0300 GMT), she said. The Mexico City subway has 12 lines and carries millions of passengers each day.

Line 12, where the accident struck, was inaugurated in October 2012 by then mayor Marcelo Ebrard, who is now foreign minister.

Writing on Twitter, Ebrard offered his cooperation with the investigation to establish the causes and responsibility for what he called a "terrible tragedy."

In one of the worst accidents on the network, two metro trains rammed into each other leaving 23 dead and 55 injured in October 1975.

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The latest incident 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.

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

The latest accident comes at a time when Mexico is struggling to cope with the coronavirus pandemic, which has left more than 217,000 people dead in the country -- one of the world's highest tolls.