The explosion caused a bridge to partially collapse. The incident, which took place near the Bologna Airport, injured at least 50 people in all, but other media outlets reported that as many as 60 or 70 people were hurt.
A truck carrying flammable cargo exploded in Borgo Panigale, located on the outskirts of Bologna, when it collided with another tanker truck just before 2 p.m., reports said.
Video on social media showed the tanker bursting into a very big mushroom cloud of orange flames and a thick plume of black smoke pouring into the sky.
The blast that was reportedly triggered by a road accident sent a huge fireball into the sky, said reports. Videos show flames shooting up in the air after the explosion, replaced later by a thick black cloud of smoke.
Fire raging on the motorway outside Bologna
In a tweet, Bologna Airport said it wasn't affected by the explosion.
Italian private Sky TG24 reported that some of the injured people had been hit by flying glass when windows in nearby buildings exploded.
In a separate incident on Monday, 12 migrant labourers died in the southern Italian region of Puglia when the van they were travelling in smashed into a lorry.