The quake, measuring 7.6 on the Richter scale hit Pakistan early on Saturday, causing widespread damage, officials and witnesses said.
"The earthquake measured 7.6 on the Richter scale and its epicentre lay 100km north of Islamabad," Meteorological department official Mohammad Iqbal said.
"We have reports of dozens of people being killed in Kashmir, Mansehra and other parts of northern Pakistan," an official said.
"There are reports of buildings collapsing in several cities in central Pakistan. We have no exact estimates but the casualty could be very high," he added.
"It is massive devastation, we are still assessing damage and casualties," said another senior government official.
The nine-storey Margalla Towers residential building in a posh area of Islamabad collapsed in a heap of shattered concrete, killing at least eight people, Ahmad Barakat, director of Aljazeera's office in Pakistan, reported.
Hundreds may be trapped under
the collapsed Margalla Towers
Eight dead bodies have so far been pulled out from the building, he said, adding that dozens, possibly hundreds, of people were still stuck in the building.
Barakat said the entrance to a nearby tower had been blocked by the debris from the collapsed building.
The second tower, also nine storeys, could also collapse any time, he added.
A number of bloodstained people could be seen trapped beneath huge stone slabs at the scene while desperate cries for help could be heard, an AFP photographer reported.
A number of people were seen
trapped beneath huge stone slabs
Military and civilian rescuers were clawing through the rubble while Interior Minister Aftab Sherpao visited the scene and comforted terrified onlookers.
One child was killed when a school fell down in the neighbouring city of Rawalpindi, Information Minister Sheikh Rashid was quoted as saying.
"A school collapsed in Rawalpindi. One child was killed and six were injured," he said. No more details were immediately available.
There were also reports of houses and structures collapsing in other northern cities and towns, although details were unclear as phone lines and roads were cut.
The military said an entire village in the Pakistani Kashmir district of Bagh was "wiped out".
"We have reports that an entire village has been wiped out in Bagh district in Kashmir," chief military spokesman Major-General Shaukat Sultan said.
"In Kashmir and the northern areas we are receiving reports of severe damage.
"The damage is widespread and we have activated troops and relief work is under way. Helicopters with medical teams have been sent from Rawalpindi and Islamabad," Sultan said.
Police said "hundreds" of people were trapped after a landslide in the northern Kaghan Valley area, a popular tourist haunt, although the report could not be confirmed.
The worst affected villages were Balakot and Gari Habibullah in Mansehra district.
"The damage is widespread and we have activated troops and relief work is under way"
Major-General Shaukat Sultan,
chief military spokesman
"We need urgent help. Rescue teams need to be sent immediately to Balakot. Hundreds of people are buried in their homes. Highways are closed," a policeman said.
The first tremor rumbled through the country at around 8.55am (0455 GMT), followed by another after a few minutes.
Many aged people said it was the worst jolt they had experienced in their life.