A van transporting oxygen gas canisters exploded in a Milan Street, injuring one person and setting nearby cars and motorbikes ablaze, local officials said.
The city's mayor, Giuseppe Sala, ruled out foul play and confirmed that no-one had died in the incident.
Mr Sala said the driver of the van had told the fire brigade that the blaze had started in his engine.
"He quickly grabbed the oxygen cylinders he was carrying ... He tried to limit the damage but then there was an explosion," the mayor said.
He added that the driver had sustained only light injuries.
Firefighters quickly extinguished the flames, which had sent clouds of thick black smoke into the sky.
Eyewitnesses said the fire appeared to have spread to a nearby building, which was evacuated.
A local school was also evacuated.
