Italian media reported the vehicle was carrying oxygen cylinders when it exploded, triggering "domino explosions".
Several vehicles are engulfed in flames, with plumes of thick black smoke rising above the wreckage.
At least five cars and four mopeds are alight, the newspaper reported, with the blaze spreading towards apartments on the street and a pharmacy.
Firefighters are arriving at the scene in the Porta Romana area of the city - which has been cordoned off by police.
A nearby school and nursing home have been evacuated.