At least 80 people have been killed so far in the quake that struck Awaran district in Balochistan province on Tuesday, according to a spokesperson of Frontier Corps, Balochistan.
About 80 injured people, including women and children, were brought to the CivilHospital in Khuzdar, a spokesman for the Balochistan government said. Reports of deaths had been received from Awaran and Turbat districts, he said.
Hundreds of mud-brick houses collapsed or were damaged in Awaran and Khuzdar areas, officials said.
Six bodies were found in Awaran bazar and four more in far flung villages of the district, said Deputy Commissioner Abdul Rasheed Baloch. "I fear more people are trapped inside collapsed houses," he said.
Ghulam Baloch, an administration official from Khuzdar, said, "Houses and shops have collapsed and the casualties could increase." Houses were also damaged in Pasni and Windar but details were awaited from these areas, Baloch said.
The army despatched 300 soldiers, rescue and medical teams and a helicopter to the worst-hit areas. Chief Minister Abdul Malik said an emergency had been declared in Awaran, seven to eight hours' drive from Quetta, the provincial capital.
Both US Geological Survey (USGS) and Pakistan's meteorological office gave the magnitude as 7.7 on the Richter Scale. USGS said five aftershocks measuring between 4.7 and 5.9 were recorded after the quake.
The quake, with its epicentre 69 km from Awaran, struck at 4.29 pm Pakistani time at a depth of 23 kilometres.
Tremors were felt in Karachi, Hyderabad, Larkana and other towns and cities of Sindh province. Reports said the quake was also felt as far away as Lahore, Rawalpindi and Islamabad.
The tremors that lasted almost a minute caused widespread panic. People rushed out of their offices and homes in Karachi, the country's largest city, and other parts of Sindh and Balochistan provinces.
National Seismic Monitoring Centre Director Zahid Rafi told PTI the tremors were of "great intensity" in Quetta and in some remote areas of Balochistan.
"The affected areas include Jaffarabad, Noskhi, Kalat, Windar, Naseerabad, Panjgur and Mastung," he said.
"The shockwaves were strong enough to even be felt in Karachi, Hyderabad, Thatta, Khairpur and Larkana," Rafi said.
In Karachi, people ran out of buildings on Chundrigar Road, the main thoroughfare in the financial district, when tremors were first felt in the city.
"I could feel the chair I was sitting on moving and shaking and there were shouts of 'earthquake, earthquake' and we all rushed out of our building," said banker Sameera.
Residents of Clifton, Defence, Gulshan-e-Iqbal and Bahadurabad too fled their homes and offices after feeling the jolts. People were seen sitting on sidewalks and footpaths, their hands raised in prayer.
TV channels beamed footage of furniture and other items moving within homes because of the quake.
Haider Ali, a resident of Clifton area in Karachi, returned home to see his wife and children running out of their high-rise apartment block.
"I rushed home because we belong to Muzaffarabad and my wife was there when the 2005 earthquake caused so much destruction and death," he said.
Long traffic jams were seen on the roads of Karachi as people left office early after the met office said there could be more aftershocks.
In October 2005, a 7.6-magnitude quake killed around 75,000 people and caused massive destruction in the Kashmir region.
In April this year, a quake that hit Karachi and parts of Balochistan killed about 40 people and destroyed or damaged hundreds of houses.