By Stephen Jewkes

FINALE EMILIA, Italy (Reuters) - Thousands of people in northern Italy slept in tents and cars overnight as more than 100 aftershocks rocked the area hit by a magnitude 6.0 earthquake that killed seven people and inflicted heavy damage to centuries-old cultural sites.

"The fear that your house will collapse on your head is great, so it was good to be able to sleep in this tent," said one man who spent the night outdoors, cold but safe, in the town of San Felice Sul Panaro.

Heavy rainfall added to the misery of about 3,000 people who had to abandon their homes and made conditions more difficult for civil protection workers.

But most residents said they were content with the relief effort. "They set up these tents very quickly. I felt safe," an elderly woman said.

Sunday's earthquake killed four factory workers who were on the night shift, an elderly woman who was hit by a beam and two women who died as a result of shock.

It also left a swathe of damage across the Emilia-Romagna region, felling ancient churches and severely damaging a 14th-century castle that had withstood wars and invasions.

The 14th-century clock tower in Finale Emilia was split vertically as if hit by a meat cleaver when the quake struck at 4:04 a.m. (0204 GMT), leaving only one half - showing the Roman numerals from seven to eleven - standing. Twelve hours later, an aftershock of magnitude 5.1 brought down the other half.

The quake hit a generally flat area in the Po River Valley that was believed to have been safe from major seismic activity.

The quake, and a bombing that killed a teenage girl in southern Italy on Saturday, prompted Prime Minister Mario Monti to cut short his trip to the United States and return to Italy.

"This is one of the times that the country should feel united and close to those who are suffering and I believe it is," he said in Chicago, announcing his decision to return early from a NATO summit.

The tremors caused the greatest loss to Italy's artistic heritage since an earthquake in 1997 ravaged the Basilica of St Francis in Assisi, where the ceiling collapsed.

The imposing 14th-century Estense Castle, symbol of the town of San Felice Sul Panaro, was severely damaged.

The tops of several of its smaller towers collapsed and there were fears that the main tower, weakened by cracks, could tumble. Centuries-old frescoes and other works of art were badly damaged in three of the town's churches.

"We have practically lost all our artistic patrimony," said Alberto Silvestri, mayor of San Felice. "Churches and towers collapsed. The theatre is still standing but has cracks."

The quake left a gaping hole in the side of the Renaissance-style town hall in Sant'Agostino, which officials said was in danger of collapsing.

Smaller aftershocks, reaching magnitude 2.5, rattled the area on Monday.

(Writing by Philip Pullella; Editing by Janet Lawrence)