In a developing story, a rescue operation is under way in Mexico City after a metro overpass partially collapsed overnight, killing at least 20 people and injuring dozens more, civil protection authorities in Mexico said.

From footage widely shared on social media, it appears that a metro overpass collapsed while a train was travelling on it. Several train carriages plunged to the ground, crushing at least one car, which was on a busy road underneath. Social media footage shows the overpass collapsing at the Olivos station on the metro's Line 12. Rescue workers and firefighters are searching for survivors after the accident which happened in the capital late on Monday evening.

According to local media, residents had reported cracks in the structure after a 2017 earthquake and 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 help coordinate the search and rescue operation. Videos on Mexican television showed train cars hanging in mid-air and footage on Milenio TV shows the overpass collapsing on to cars on a road below. There are fears that the death toll could rise even further as it is believed that people may still be trapped under the rubble.

Mexico City’s metro is one of the world's largest rapid transit systems, second only to New York in terms of passenger volumes, carrying about 1.6 billion passengers a year. Line 12, where the incident occurred, is the newest addition to the capital’s metro and officially opened in 2012.

Last year, one person was killed with more than 40 injured after two metro trains collided in the city at the Tacubaya station. In 2015, a train that failed to stop on time crashed into another at the Oceania station, injuring 12 people.