Dozens of people are dead as rescuers work to free motorists buried under the rubble of a motorway bridge that collapsed amid torrential rain in the Italian city of Genoa.

A 50-metre high section of the bridge, including one set of the supports that tower above it, crashed onto the roof of a factory and other buildings on Tuesday night (AEST), crushing vehicles below and plunging huge slabs of reinforced concrete into the nearby riverbed.

A large-scale rescue operation, including sniffer dogs, was searching under the many tonnes of concrete and metal rubble  in the industrial area, railway track and the river where much of the bridge landed.

The exact extent of the death toll remained unclear, with officials in the Liguria region, which includes Genoa, on Wednesday morning (AEST) saying 26 people were confirmed to have lost their lives.

The office of the Liguria governor earlier stated the death toll had climbed to 35.

At least 16 survivors have been taken to hospital.

Helicopter footage on social media showed trucks and cars stranded on either side of the 80-metre long collapsed section of the Morandi Bridge, built on the A10 toll motorway in the late 1960s. One truck was shown just metres away from the broken end of the bridge.

More than 30 vehicles were on the bridge when it collapsed. Photo: Getty

Motorist Alessandro Megna told RAI state radio he had been in a traffic jam below the bridge and seen the collapse.

“Suddenly the bridge came down with everything it was carrying. It was really an apocalyptic scene, I couldn’t believe my eyes,” he said.

Transport Minister Danilo Toninelli told Italian state television the disaster pointed to a lack of maintenance, adding that “those responsible will have to pay”.

But Stefano Marigliani, the motorway operator Autostrade’s official who is responsible for the Genoa area, told Reuters the bridge was “constantly monitored and supervised well beyond what the law required.”

He said there was “no reason to consider the bridge was dangerous”.

Restructuring work was carried out in 2016 on the 1.2 km-long bridge, first completed in 1967. The motorway is a major artery to the Italian Riviera and to France’s southern coast.

A witness captured these images from his office moments after the collapse of a bridge in Genoa, Italy pic.twitter.com/GigjmgPX3J

— euronews (@euronews) August 14, 2018

Angelo Borrelli, head of the Italian civil protection agency, said there were 30-35 vehicles on the bridge when the middle section came down, including three large trucks.

Mr Borelli said 13 people had been hospitalised, including five in a critical condition.

Breaking: Italy’s ambulance service claims that dozens of people have died after a bridge collapsed in Genoa, Italy. pic.twitter.com/h10t2Dime7

— PM Breaking News (@PMBreakingNews) August 14, 2018

A fire service spokesperson told AFP that the bridge had mostly fallen on rail tracks 100m below and that “cars and trucks” had fallen with the rubble.

Genoa is located between the sea and the mountains of northwestern Italy.

-with agencies