A metro overpass has collapsed in Mexico City as a train was travelling over it, killing at least 24 people, including children, the mayor said.

Two train carriages were seen hanging from the structure, above a busy road. At least 79 people were injured, and seven are in a serious condition.

One person trapped in a car underneath the wreckage was pulled out alive.

This is the deadliest incident in decades in the city’s metro system, one of the busiest in the world.

Officials refused to speculate on the cause the disaster which sent two carriages crashing into passing traffic on the street below on Monday night. The city’s mayor, Claudia Sheinbaum, promised a thorough investigation by an outside firm and the federal prosecutor’s office – though she stood by the embattled director of the metro, Florencia Serranía.

Speaking at a news conference alongside the mayor, President Andrés Manuel López Obrador offered his condolences to the relatives of the victims and said that “nothing would be hidden” from the Mexican people.

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.