At least 20 people have been killed after a metro overpass partially collapsed in Mexico City, authorities said.

The city’s civil protection agency added that almost 70 people were injured in the accident in the south of Mexico’s capital, which happened at 10.30pm local time.

Mexico City Mayor Claudia Sheinbaum, who had rushed to the scene of the accident, said a “support beam gave way”.

She said 34 of the injured have been taken to hospital.

The scene of the incident in Mexico City
The scene of the incident in Mexico City (Marco Ugarte?AP)

The collapse led to a train falling downwards and cars have been trapped underneath the rubble.

Video showed at least one broken train along with dozens of rescuers searching through the remnants of the collapsed overpass.

The overpass was about five metres above the road in southern Mexico City.

Hundreds of police and firefighters cordoned off the scene in the southern borough of Tlahuac, as desperate friends and relatives gathered outside the security perimeter.

People wait for news at a barricade near the scene in Mexico City
People wait for news at a barricade near the scene (Eduardo Castillo/AP)

The accident happened on the metro’s Line 12, the construction of which has been plagued by complaints and accusations of irregularities.

Mexican foreign minister Marcelo Ebrard wrote on Twitter: “What has happened on the metro today is a terrible tragedy.”

He added: “Of course the causes should be investigated and those responsible should be identified. I repeat that I am entirely at the disposition of authorities to contribute in whatever way is necessary.”

Soldiers hold stretchers as they wait to help in the rescue effort
Soldiers hold stretchers as they wait to help in the rescue effort (Marco Ugarte/AP)

The Mexico City metro, one of the largest and busiest in the world, has had at least two serious accidents since its inauguration half a century ago.

In March of last year, a collision between two trains at the Tacubaya station left one passenger dead, injuring 41 people. In 2015, a train that did not stop on time crashed into another at the Oceania station, injuring 12 people.