Italian families on holiday and people from at least three different countries were among the dozens of fatalities that occurred when a highway viaduct collapsed in the northwestern city of Genoa.

A large section of the much-trafficked Morandi Bridge, built in the 1960s, gave way amid a torrential downpour shortly before noon on Tuesday, sending several cars and trucks crashing onto a railway, a riverbed and a couple of warehouses that lay 45 metres below.

Speaking to reporters on Wednesday evening, Prime Minister Giuseppe Conte put the provisional death toll at 39.

Among the victims were four French nationals. Also reportedly on the list of confirmed victims were two Romanian citizens, and three Chileans who were long-time residents of Italy.

The Italian victims included families, couples, and groups of friends on their way to seaside destinations ahead of Italy's "Ferragosto" national holiday, which this year fell on Wednesday.

The town of Campomorone, located inland from Genoa, lost four of its own, including a couple in their early 40s and their little boy who were reportedly on their way to a holiday on the island of Sardinia, according to the town's Facebook page.

Another Italian couple who fell to their deaths along with their two teenaged children were from the Piedmont city of Pinerolo. Its administration wrote on Facebook that it is "overwhelmed by so much tragedy" and expressed its sympathies to "the families of our fellow citizens who passed way in Genoa."

Four young friends from the Sicilian town of Torre del Greco who were on their way to Barcelona also died under the bridge, according to local news website Vesuvio Live.

Hundreds of rescuers worked through the night and were continuing to dig through the rubble for survivors more than 24 hours after the disaster, while the government declared a state of emergency for the Liguria region whose capital is Genoa.