Mexico will find out who was responsible for the overpass collapse that killed at least 23 people and injured dozens more when a train on Mexico City’s newest metro line plunged onto a busy road below, the government says.

Mexican President Andres Manuel Lopez Obrador said the investigation should be done quickly and that nothing should be hidden from the public.

“There’s no impunity for anyone,” he told a news conference.

The crash has raised questions about safety on one of the world’s busiest metro systems, which spreads across an urban sprawl home to more than 20 million people.

Bridge structures collapsed trapping victims in the rubble. Photo: Getty

The city has been governed since the turn of the century by former mayor Lopez Obrador and his allies.

Firefighters using heavy chains to stabilise the site pulled bodies and survivors from the wreckage.

City authorities said 79 people were injured, including three children, and survivors were taken to nearby hospitals in the south of the capital.

Video on social media showed the moment when the overpass plummeted onto a stream of cars near the Olivos station in the southeast of the city about 10.30pm on Tuesday, sending up clouds of dust and sparks.

Monserrat, 26, said she was at the back of the train wagon when she heard a loud noise and the lights went out.

“Everybody screamed and we fell on top of each other, I was thrown on top of everybody, I fell on everybody,” she told Mexican radio, speaking from the Belesario Dominguez hospital where she was receiving treatment for an injured rib.

Medical personnel are ready to attend to people after the transport disaster. Photo: Getty

Workers on Tuesday hoisted one of two train cars dangling from the bridge using several tall cranes and slowly lowered it close to the ground.

Twisted pieces of metal could be seen inside.

It was the second serious accident this year, after a fire at a central control building knocked out service on several lines for weeks.

The overpass that collapsed was part of Linea 12, an addition to the network finished less than a decade ago and long plagued by allegations of corruption.

Investigations will be carried out by both the attorney general’s office and an external auditor, the government said.