A powerful earthquake shook Italy's densely populated industrial northeast early Sunday, killing at least six people and reducing homes, factories and historic buildings to rubble.

Emergency services said dozens had been injured in the magnitude 6.0 quake, which struck in the middle of the night, sending thousands of people running into the streets in towns and cities across the Emilia Romagna region.

Prime Minister Mario Monti was to return early from the United States, where he was attending a NATO summit, as Italy reeled from the double shock of the quake and a deadly school attack that took place on Saturday.

Emergency workers were sifting through the rubble of collapsed buildings for victims hours after the quake and several aftershocks struck at 0200 GMT.

European Commission chief Jose Manuel Barroso expressed his "profound sadness" and said Brussels was "ready to provide swiftly any assistance that may be requested."

Four of the dead were night-shift workers in factories which collapsed, including two who were crushed when the roof of a ceramics factory caved in in the town of Sant'Agostino.

A 37-year-old German woman and another woman aged over 100 reportedly died from shock.

The quake caused "significant" damage to historic buildings as it rattled the cities of Bologna, Ferrara, Verona and Mantua, Italy's culture ministry said.

"According to first reports, damage to the cultural heritage is significant," the ministry said, adding that it was carrying out "more detailed verifications with firemen and the civil defence service."

Italian television showed many historic buildings, including churches, reduced to rubble. Cars were crushed under falling masonry, and the Civil Protection Agency evacuated hundreds of elderly and vulnerable people to makeshift communal shelters in Finale Emilia and towns near the epicentre.

Hospitals were evacuated as a precautionary measure and about 3,000 people have been evacuated from their homes.

Warehouses storing more than 300,000 wheels of Parmesan and Grana Padano, a similar cheese, with an estimated value of more than 250 million euros ($320 million), also collapsed, an industry official said.

The roof of a recently renovated sixth-century chapel in San Carlo, near Ferrara, caved in, exposing statues of angels to the elements.

Claudio Fabbri, a 37-year-old architect, told AFP the restoration had taken eight years. "Now there's nothing left to do," he said despondently.

People were out in the street, fearful of going indoors, as the odour of gas hung in the air.

Retired electrician Claudio Bignami, 68, said: "I went out because I felt the house moving. Furniture was falling over. It was chaos. Everyone was running in every direction."

Aldra Bregoli, 73, who had pulled on a sweater over her nightgown, said: "I had to get out quickly. I can't go back home. I'm afraid."

Authorities said the quake's epicentre was the commune of Finale Emilia, 36 kilometres (22 miles) north of Bologna, at a depth of only 5.1 kilometres.

One of the men killed in the ceramics factory collapse, Nicola Cavicchi, 35, "wanted to go to the seaside but because of the bad weather forecast he decided to go to work to replace a colleague who was sick," a family member told local media.

A 29-year-old Moroccan man was killed by a falling girder when a factory building collapsed in the small town of Ponte Rodoni di Bondeno.

The body of a fourth night-shift worker was found in the early afternoon under fallen masonry at a factory in a nearby village.

In Finale Emilia, firefighters rescued a five-year-old girl who was trapped in the rubble of her house after a rapid series of phone calls between a local woman, a family friend who was in New York and emergency services.

The region shaken by the quake is Italy's industrial heartland but also home to priceless architectural and art treasures. The historic centre of Ferrara is classified as a UNESCO World Heritage site.

A 5.1 magnitude aftershock struck Sunday afternoon, forcing the collapse of several structures already weakened, with a firefighter left seriously injured after falling from a wall.

Yet in a show of calm nerves, officials opened polls as planned for the second round of local elections in the cities of Piacenza, Parma, Budrio and Comacchio.

"Italy is a very quake-prone country," said seismologist Enzo Boschi.

In March 2009, a 6.3 magnitude quake devastated the central city of l'Aquila, killing some 300 people and leaving tens of thousands homeless.