The incident sent around 20 cars and three heavy trucks tumbling as an 80-metre stretch of elevated highway fell away.
Italian transport minister Danilo Toninelli said the collapse of the Morandi Bridge was an “immense tragedy”.
“I’m following with great apprehension what has happened in Genoa, which appears to be an immense tragedy,” he said in a statement posted on social media.
Italian news agency Ansa said authorities believe structural weaknesses may have caused the collapse.
The viaduct, near the port city of Genoa, was built for the A10 motorway in the 1960s, with restructuring work reportedly carried out on it in the 2016.
Local broadcasters say the collapse happened near an industrial zone and that firefighters are worried about gas lines potentially exploding from the knock-on effects of the collapse.
The country’s interior minister Matteo Salvini said around 200 firefighters were responding to the accident and that the government was “following minute-by-minute the situation”.
Genoa bridge: Aerial video shows aftermath of Italy motorway collapse
“Around 30 people are dead and many others are seriously injured,” Mr Salvini told reporters in Sicily.
The office of Giuseppe Conte, the Italian prime minister, said the premier would be travelling to Genoa in the evening and would stay there through Wednesday.
Italian bridge collapses: part of highway gives way in Genoa
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Defence minister Elisabetta Trenta announced that the army was ready to offer manpower and vehicles to assist with the rescue operations following the disaster.
The Autostrada 10 runs along the northwestern Italian coast, connecting Genoa with the city of Ventimiglia.
It is a component part of the wider European route E80 which stretches from Portugal to eastern Turkey, notably connecting northern Italy to the south of France.
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