Up to 40 vehicles were travelling on the stretch at 11.30am when it was reportedly struck by lightning during torrential rain, causing lorries and cars to plummet 150ft as the concrete crumbled.
Photographs showed traffic still travelling along the A10 toll highway, with drivers unaware of the disaster ahead – and one lorry perched yards from the edge of the destroyed bridge.
Around 200 firefighters and sniffer dogs searched for survivors among the wreckage.
Four people were pulled out alive and rescuers last night said a further 10 were unaccounted for.
Italy’s department for civil protection said 35 cars and three heavy vehicles were under the bridge and a transport official said eight people were seriously injured.
French firefighter Patrick Villardry, who came from Nice to help the rescue effort, said: “We have to search under the wreckage of buildings, but there are thousands of tons of concrete.”
Ponte Morandi, built in the 1960s, was undergoing repairs when the tragedy happened.
One rescue worker described the scene as “hell” and a witness said it “seems like a bomb has hit”.
Genoa resident Elizabeth told the BBC: “The state of the bridge always concerned us. Nobody has ever crossed that bridge with a light heart.
“Everybody has always done it praying that the bridge wouldn’t fall down. Today that happened.”
The exact cause of the collapse was not yet clear, but transport minister Danilo Toninelli said the “immense tragedy” showed the dilapidated state of the country’s infrastructure and a lack of vital maintenance, adding that “those responsible will have to pay”.
Deputy prime minister Matteo Salvini said: “The more I think about the deaths of Genoa, the more I get angry. Those responsible for this disaster, with names and surnames, will have to pay, pay everything, pay dearly.”
Stefano Marigliani, a director of Autostrade, which manages the section of road, insisted the tragedy was impossible to predict.
He said: “The collapse was unexpected and unpredictable. The bridge was constantly monitored, even more than was foreseen by the law. There was no reason to consider the bridge dangerous.”
The highway connecting France and the Italian Riviera is popular with tourists.
Traffic would have been heavier than usual as Ferragosto, a public holiday, falls today.
A family believed to be on holiday was found in the rubble with suitcases inside their car.
Yesterday’s incident is the latest in a string of Italian bridge collapses.
The country is prone to seismic activity, but its infrastructure is showing the effects of economic stagnation.
Last March a couple died when a motorway flyover collapsed near Ancona. A pensioner died in 2016 when a bridge fell onto a major road between Milan and Lecco.