The AP reports a bridge has partially collapsed.
Videos posted on social media showed a giant fireball and smoke emanating towards the sky, as police deployed firefighters to the spot.
More than a dozen people were seriously injured, local media said. The police blockaded the motorway in Bologna's Borgo Panigale area - where the blast occurred.
Most of the videos were taken from inside passing cars where people were able to shield themselves from the blast and subsequent heat.
Firefighters are now battling the flames at the scene on the bypass in Borgo Panigale, according to local reports.More news: The far too early Liverpool starting XI prediction
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A video shot from a parked vehicle at the moment of the explosion appeared to show someone being engulfed by flames when the tanker exploded.
Google Maps/HuffPost UK The explosion took place in the northern Italian city of Bologna on Monday.
Italian firefighters tweeted that they had sent multiple rescue and emergency squads, including one from their urban search and rescue team.
Italian media reported that numerous injured were hit by debris and shattered glass after the explosion.