At least 20 people have been killed and dozens injured after a metro overpass collapsed in Mexico City while a train was travelling on it, officials say.

Several train carriages plunged to the ground, crushing at least one car, which was on a busy road underneath.

Rescue workers and firefighters are searching for survivors after the accident in the capital late on Monday.

Dramatic footage has emerged showing the overpass collapsing at the Olivos station on the metro's Line 12.

Local residents had reported cracks in the structure after a deadly 2017 earthquake, according to local media. Mexico's El Universal newspaper says transport authorities made repairs following the reports.

Mexico City Mayor Claudia Sheinbaum said 20 people were confirmed dead, after she arrived at the crash site to co-ordinate the search and rescue operation.

About 70 people were injured, local emergency services said.

There are fears that the death toll could rise further as some people may still be trapped under the rubble.

"We only heard a loud thunder, and everything fell apart," 26-year-old Mariana, who survived the crash, told El Universal.

"There were many people standing and sitting in the carriage," she said, adding that she hit the carriage's roof.

One eyewitness told Mexico's Televisa TV network: "Suddenly I saw that the structure was shaking."

"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," the witness said.

Mexico City's metro is one of the world's largest rapid transit systems, carrying about 1.6bn passengers a year.

Line 12 was the newest addition to the capital's metro. It officially opened in 2012.

Last year, one person was killed and more than 40 injured in a collision between two metro trains in the city.

In 1975, two underground trains collided, killing 31 people.