* Seven people killed in Sunday's earthquake
* Area hit by more than 100 aftershocks, some strong
* PM Monti cuts short trip to NATO summit in Chicago
* Some 4,000 homeless, 200 million euros in farm damage (Adds fresh quotes, agricultural damage)
By Stephen Jewkes
FINALE EMILIA, Italy, May 21 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 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 of shock.
It also caused an estimated 200 million euros' damage to agriculture and left a swathe of destruction across the Emilia-Romagna region, felling ancient churches and severely damaging a castle that had withstood wars and invasions for seven centuries.
A 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 rest.
"I had to come here. They haven't said when we can go back because the aftershocks are continuing," said Michelina Salvatico, a resident of Finale Emilia who was moved to a sports centre after her house was damaged.
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 a trip to the United States.
"This is one of the times that the country should feel united and close to those who are suffering, and I believe it is," Monti 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.
PROSCIUTTO AND PARMESAN
"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 town hall in Sant'Agostino, which officials said was in danger of collapsing.
Smaller aftershocks, reaching magnitude 2.5, continued to rattle the area on Monday.
The damage to agriculture and livestock, in what is one of Italy's most fertile food producing regions, was estimated to be at least 200 million euros, the farmers group Coldiretti said.
Stables, barns and animal pens were damaged and some 400,000 large wheels of the area's world-famous Parmigiano Reggiano and Grana Padano cheeses fell from shelves in warehouses where they were undergoing seasoning.
The quake could also affect milk and ham production in the area - famed for Parma ham - because of deaths and injuries suffered by cows and pigs, Coldiretti said. (Writing By Philip Pullella; Editing by Robin Pomeroy)