Security camera footage showed the carriages plunging toward the ground in the south of the city

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.

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

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"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.

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