Numerous victims had been hit by flying glass when windows in nearby buildings exploded, a local television network reported.
Doctors said 120 people were treated and 18 were hospitalized for injuries that included severe burns.
Video captured by a witness shows the explosion and a plume of smoke filling the sky. It also shut down a highway north of the city that is a key route between northern Italy and the Adriatic Coast and other points south, including Florence and Rome, the capital.
A huge gaping hole could be seen in the raised highway next to the tanker as the fire raged.
The ones hurt by glass included 11 carabinieri from a barracks near the blast site who were preparing to respond to the accident, officials said.
A video taken from a fire service helicopter as firefighters arrived to extinguish the blaze and secure the area.
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