BONDENO, Italy - A strong earthquake rocked a large swathe of northern Italy early on Sunday, killing at least four people, injuring dozens and seriously damaging historic buildings such as churches, bell towers and a mediaeval castle.

The quake, which the US Geological Survey recorded at magnitude 6.0, struck at 4:04 am while most people were sleeping, and thousands ran into the streets in their night clothes in panic.

"I ran out in my underwear," one man told Italian television.

The epicentre of the quake, the strongest to hit Italy in three years, was in the plains near Modena in the Emilia-Romagna region of the Po river valley, and the tremor was felt as far west as Liguria, bordering France, and the Friuli region bordering Slovenia.

The roof of the cathedral in Mirandola collapsed. "Our school children were to receive their first communion here this morning. If it had happened then it would have been a disaster," the local priest said.

Also badly damaged was the 14th century Estense Castle in the town of San Felice Sul Panaro.

The tops of several of the smaller towers of the famous mediaeval castle, the town's biggest attraction, collapsed and there were fears that the main tower could crumble. Three of the town's churches were severely damaged.

One person, believed to be a Moroccan man working a night shift in a polyester factory, died when he was hit by falling debris, and two men, also on the night shift, were killed when part of a modern ceramics factory made of steel collapsed in the town of Sant' Agostino.

"He wasn't supposed to be there. He changed shifts with a friend who wanted to go to the beach," the mother of one of the victims told state television.

The lifeless body of a fourth victim was spotted under rubble in another factory.