Rescue workers say about 10 vehicles were involved when a 200m (650ft) section of the Morandi Bridge collapsed during a sudden and violent storm.
The Italian Ambulance Service says there are "dozens of dead", local news agency Adnkronos reported.
One person has been rescued alive from the rubble and flown to hospital by helicopter, ANSA news agency reported.
Video showing parts of the structure falling into the water has been posted to Twitter by local police.
What appears to be flashes of electricity are visible as concrete columns shatter and drop.
Around 200 firefighters are responding to the accident and have told the Associated Press they are concerned about gas lines.