A tanker truck carrying flammable material exploded on a road near the northern Italian city of Bologna on Monday afternoon.
At least two people have died and up to 70 were injured by the explosion which caused a raised roadway to partially collapse.
Some people were reportedly hit by flying glass as windows shattered in nearby buildings.
Italian police said between 60 and 70 people had been injured, some with severe burns, in the midday accident on a major route north of the city.
Officials did not immediately know what kind of explosives were in the tanker, but the news agency ANSA said it was liquefied petroleum gas.
The explosion reportedly came after a traffic accident.
Videos show flames shooting up in the air after the explosion, replaced later by a thick black cloud of smoke. Aerial photos showed a gaping hole in the raised highway next to the tanker.
Bologna Police’s official Twitter account said: “Due to accident on the highway, there was an explosion resulting in a fire via Marco Emilio Lepidus.
“Impaired circulation, not only on the highway, but also in the area of Borgo Panigale.”
The explosion has sparked panic on Twitter as users shared pictures and footage of huge fireball.
One Twitter user said: “I was literally looking out my window and suddenly there was a lot of fire in the air.”
Another questioned: “What’s with the explosion near Bologna airport in Italy?”
Terrified residents reported their houses “shook” when the vehicle exploded at around 12.50pm. Fire and ambulance crews rushed to the scene at the scene and the motorway was closed in both directions.
Several car dealerships were said to be in flames, and the windows of houses blown out. One local media outlet reported two police officers were caught in the blast as they directed traffic from a previous road accident. Italian television channel Sky TG24 said the flames fell to the lower level, setting off secondary explosions in a car park below.
It said some of the injured had been hit by flying glass when windows in nearby buildings exploded. Firefighters said the flames were extinguished some three hours after the explosion and authorities said a major highway junction had been closed north of the city due to the accident.
The junction connects two major highways linking northern Italy with the Adriatic coast, a popular destination as Italy heads into its major summer holiday next week.
Witness Marco Rosadini was in a nearby restaurant when the incident occurred. He told La Repubblica that he initially thought it was an attack.
“With the explosion, the glass roof of the restaurant started to collapse. People started to run away,” he said. “We were seated, at one point there was a very loud explosion and everything started to come down in. I looked outside and it seemed to me [there was] a whole wall of flames. There were seven to eight minutes of explosions: I did not really think about an accident, I had thought about an attack.”
Deputy Prime Minister Matteo Salvini paid tribute to the emergency services that responded to the blast.
“My thoughts are with the victims and the injured of the terrible explosion of Borgo Panigale, and a heartfelt thanks to the 100 firefighters who promptly intervened on the spot,” he wrote on Facebook.
The city’s airport is around 3.7 miles north-west of the city centre in northern Italy’s Emilia-Romagna region.
Management at the airport said there was no disruption to flights. In a separate incident yesterday, 12 migrant labourers died in the southern Italian region of Puglia when the van they were travelling in smashed into a lorry.