The US Geological Survey's website puts the magnitude at 6.6.
Initial reports on the magnitude of the tremors varied – while USGS and Italian media first talked of a 7.1 earthquake, the European-Mediterranean Seismological Center (EMSC) said the tremor was magnitude 6.5 or 6.6.
Italy Civil Protection reported buildings collapsing in a number of locations following the tremors.
“No deaths have been reported, but there are a number of people injured,” Civil Protection chief Fabrizio Curcio told a news conference, as cited by Reuters, adding that at least one person was in serious condition.
Curcio added that 15,000 people had been left without electricity or drinking water, while roads were severely damaged, leaving access only to emergency vehicles. In addition, thousands of homes have been destroyed and people forced to seek shelter.
“There are between 10,000 and 100,000 people who will need to be assisted,” said Luca Ceriscioli, the president of the Marche region, adding that if the seismic activity does not stop, “you are likely to get to 100 thousand displaced people.”
The USGS says the quake was centered 6 km (3.7 miles) north of Norcia, a town in the province of Perugia. The epicenter lay some 10 kilometers deep.
Norcia, famed for its Benedictine monastery and its cured meats, is home to some 5,000 people.
According to RAI Radio, at least two buildings collapsed there - the Basilica di San Benedetto, the 14th century cathedral in one of the city's main piazza, and the Cathedral of Santa Maria Argentea.
“It's as if the whole city fell down,” Norcia city assessor Giuseppina Perla told the ANSA news agency.
Authorities said many towns and villages already battered by the 6.2 quake in August had seen further damage.
"This morning's quake has hit the few things that were left standing. We will have to start from scratch," Michele Franchi, the deputy mayor of Arquata del Tronto, told RAI TV.
According to the mayor of Ussita, a commune in the Marche district with around 450 inhabitants, 90 percent of the buildings in the area were brought down by the quake. While in the town of Amatrice the Church of Saint Augustine lost its bell tower.
Aftershocks with magnitudes of up to 4.5 rocked the region throughout the whole day and are expected to go on for a week.
Italians who live in the areas affected by the quake rushed to the web, posting videos of shaking furniture inside their homes.
According to those on Twitter, tremors were felt in Perugia, Rimini, Abruzzo, Naples and Rome, where the circulation of the metro-system was briefly suspended on lines A and B.
On Wednesday, central Italy was already rocked by a 6.0 magnitude earthquake, which left several thousand people.
Italy's National Commission for the Forecast and Prevention of Major Risks (CGR) cautioned Friday that more powerful earthquakes could hit the region in the nearest future, identifying at least three areas at risk for further seismic activity.
"There is no current evidence that the (seismic) sequence underway is coming to an end," the commission warned.
This week's quakes come mere two months after almost 300 people were killed in the region by a quake that leveled several small towns.