A powerful earthquake with a preliminary magnitude of 6.6 on Sunday rocked the same area of central and southern Italy hit by quake in August and a pair of aftershocks last week, sending already quake-damaged buildings crumbling after a week of temblors that have left thousands homeless.

The earthquake was believed to be the strongest to strike the country since 1980. A 6.9-magnitude quake in southern Campania that year killed some 3,000 people and caused extensive damage.

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Some 20 people suffered injuries, most of them minor, investigators said. However, numerous buildings that had resisted the previous quakes collapsed. Crews were responding with helicopters as rockslides blocked many roads, the head of Italy's civil protection agency, Fabrizio Curcio, told reporters.

Closest to the epicenter was the ancient city of Norcia, the birthplace of St. Benedict, the father of monasticism, and famed for its Benedictine monastery. Witnesses said the 14th century St. Benedict Cathedral collapsed in the quake, with only the facade still standing.

"It seemed like a bomb exploded inside the house," the deputy mayor of Norcia, Pierluigi Altavilla, told Sky TG24.

Residents already rattled by a constant trembling of the earth rushed into piazzas and streets after being awoken by the 7:40 a.m. quake.

Many people still had been sleeping in cars or evacuated to shelters or hotels in other areas after a pair of strong jolts on Wednesday. Curcio said 1,300 had been evacuated to the coast, and more would follow.

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Pope Francis told crowds at St. Peter's Square, "I'm praying for the injured and the families who have suffered the most damage, as well as for rescue and first-aid workers." Cardinal Gualtiero Bassetti told priests in the Umbria region to hold Mass outdoors.

The quake struck a cluster of mountain towns, many of historic significance, already reeling from last week's pair of aftershocks to an August earthquake that killed nearly 300: Norcia, Visso, Castelsantangelo sul Nero and Preci.

The head of the civil protection authority in Italy's Marche region, Cesare Spuri, said there were reports of buildings collapsing in many cities.

"We are trying to understand if people are under the rubble," Spuri said.

Television images in the minutes after the quake showed nuns rushing out of their church and into Norcia's main piazza as the clock tower appeared ready to fall. One nun had to be carried by firefighters, while another was supported as she walked. Later, priests and nuns prayed in the square amid the rubble.

"It's as if the whole city fell down," Norcia city assessor Giuseppina Perla told the ANSA news agency.

The town closest to the quake's epicenter, Norcia is the birthplace of St. Benedict, the father of monasticism and has suffered a series of earthquakes over its history. The cathedral was built over Benedict's birthplace.

The monks of Norcia confirmed the collapse of the St. Benedict cathedral in a letter launching an immediate fundraising campaign to rebuild.

The current superior, who signed the letter to supporters as the Rev. Benedict, reported the cathedral was "flattened," and that monks were combing the city to help where needed.

"May this image serve to illustrate the power of this earthquake, and the urgency we monks feel to seek out those who need the sacraments on this difficult day for Italy," he wrote.

The hilltop town of Camerino, some 60 kilometers from Ancona, suffered new building collapses but no reports of injuries. City spokesman Emmanuele Pironi said the main fire hall had been rendered uninhabitable and that they had transferred to a warehouse.

"An hour and a half after the quake, we can be reassured," Pironi told The Associated Press.

Pironi said most of the area's 9,000 university students had left after the town's historic center was closed due to danger of collapses last week, and some of the 7,000 residents had been moved to hotels near the coast or to shelters nearby. Few remained in their homes.

The mayor of quake-hit Ussita said a huge cloud of smoke erupted from the crumbled buildings.

"It's a disaster, a disaster!" Mayor Marco Rinaldi told ANSA. "I was sleeping in the car and I saw hell."

In Arquata del Tronto, which had been devastated by the Aug. 24 earthquake that killed nearly 300 people, Arquata Mayor Aleandro Petrucci said, "There are no towns left."

"Everything came down," he said.

New collapses also were reported in Tolentino, where the news agency ANSA said three people were extracted from the rubble.

The quake was felt throughout the Italian peninsula, with reports as far north as Bolzano near the Austrian border and as far south as Bari in the Puglia region. Residents rushed into the streets in Rome, where ancient palazzi shook, swayed and lurched for a prolonged spell.

Austria's governmental earthquake monitoring organization said the quake was felt to varying degrees in the east and south of the country and all the way to the city of Salzburg. It says that at its strongest, residents in upper floors noticed a swaying sensation and a slow swinging of hanging objects.

The quake sent boulders raining onto state highways and smaller roads, forcing closures throughout the quake zone that was impeding access to hard-hit cities such as Norcia. Traffic was being diverted to other roads.

The Salaria highway, one of the main highways in the region, was closed at certain points as it was after Wednesday's quakes.
In addition, Italy's rail line said some local lines in Umbria and Le Marche were closed as a precaution.

The European-Mediterranean Seismological Center put the magnitude at 6.6 or 6.5 with an epicenter 82 miles northeast of Rome and 42 miles east of Perugia, near the epicenter of last week's temblors. The U.S. Geological Survey put the magnitude at 6.6.

The German Research Centre for Geosciences put the magnitude at 6.5 and said it had a depth of 6 miles, a relatively shallow quake near the surface but in the norm for the quake-prone Apennine Mountain region.

The Associated Press contributed to this report.