Part of the bridge remains intact as the middle falls away, believed to be 100 yards of motorway.
He said the collapse had killed at least 22 people but the number of dead would climb.
This frame taken from a video released by the Italian firefighters, shows firefighters working on the site of the collapsed Morandi highway bridge in Genoa, northern Italy, Aug. 14, 2018.
La Repubblica journalist Antonello Guerrera tweeted that emerging reports from the scene paint a "catastrophic" picture, with emergency services racing to reach victims buried under rubble or trapped in vehicles.
The day marks the high point of the Italian summer holiday season when most cities and businesses are closed and Italians head to the beaches or the mountains, which meant traffic was heavier than usual on the Genoa highway. The bridge collapsed after being hit by lightning, some eye witnesses said on Tuesday.
Images emerged later of an air ambulance being loaded with one of the injured near the bridge, which had recently been undergoing repiars.
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"I'm following with the utmost apprehension what is happening in Genoa and what looks like it could be an huge tragedy", Danilo Toninelli said on Twitter.
Restructuring work on the bridge, which was 1.2 km long in total, was carried out in 2016.
Borrelli said at a news conference in Rome that 20 people were killed and 13 injured.
The section that gave way at approximately 11.30am local time was dozens of metres in length, reports the BBC, and cars and trucks were on it when it fell.
Italian Prime Minister Giuseppe Conte has described the disaster as "an incredible tragedy".
Salvini said 200 firefighters and other rescue personnel are on the scene.
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ANSA said authorities suspected a structural weakness caused the collapse. "The bridge is a viaduct that runs over shopping centers, factories, some homes, the Genoa-Milan railway line and the Polcevera River".
On Twitter he said: "we are following minute by minute the situation of the bridge collapse in Genoa".
A view of the collapsed Morandi highway bridge in Genoa, northern Italy on Aug 14, 2018.
It is part of the major A10 highway that connects Italy with France. The portion of highway that collapsed was about 200 yards long.
One truck can be seen still perched at the end of the surviving section.
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