Official sources in Quetta, the capital of Balochistan, said that about 350 people were killed by the 7.7-magnitude quake, with a majority of deaths reported from the worst-hit Awaran district.
More people were feared to be buried under debris in far-flung areas which rescue teams had entered only on Wednesday because of the difficult terrain and lack of roads.
"I fear a further increase in the number of dead," said Balochistan government spokesman Jan Buledi.
Rescue workers were yet to reach survivors in some areas since the quake affected communication systems in remote Awaran district.
Maj Gen Muhammad Saeed Aleem, head of the National Disaster Management Authority, told reporters in Quetta he had information of 271 deaths and 246 injured. Other officials said 327 bodies were found in Awaran and Kech districts.
According to official estimates, over 300,000 people in six districts were affected by the quake that hit yesterday afternoon. Many do not have access to food, drinking water or shelter and the situation was exacerbated by the hot weather.
Awaran's population is scattered over an area of more than 21,000 sq km. More than 60,000 people are believed to have lived within 50 km of the epicentre.
Balochistan chief minister Abdul Malik Baloch declared an emergency in Awaran and five other districts located near the epicentre.
Residents of Awaran said no building in the town, including hospitals, schools and government officials, had remained intact.
Over 1,000 Army and Frontier Corps personnel are involved in rescue operations and military officials said eight tonnes of food and medicines and six helicopters had been moved to the affected areas.
The navy dispatched a contingent with relief goods to Awaran. A naval spokesperson said a medical camp had been established in Gwadar.
The Balochistan government sent 1,000 tents, 500 food bags, medicines and 15 ambulances towards Awaran, said spokesman Jan Buledi.
Buledi said the quake affected six districts and Awaran was the worst-hit area as hundreds of mud houses were destroyed.
"We are seriously lacking medical facilities and there is no space to treat injured people in local hospitals. We are trying to shift seriously injured people to Karachi in helicopters," he said.
Footage on television showed scores of collapsed homes and people sitting in the open beside piles of mud and bricks.
The quake was so powerful that it prompted a new island to rise from the sea off Pakistan's southern coast.
The tremors caused the seabed to rise and create a small, mountain-like island about 600 meters off the Gwadar coastline along the Arabian Sea.
A large number of people gathered near the coast to see the island.
"The island popped up soon after the earthquake. Our staff in Gwadar has reported that the island is about one-and-half kilometres away from the coast," said Asif Inam, the Principal Scientific Officer of the National Institute of Oceanography.
This was the third time in 15 years that such a phenomenon has occurred along the Balochistan coast.
The quake was felt as far away as Karachi, Lahore and New Delhi.
Aftershocks continued to be felt in Pakistan, and the latest one measuring 4.7 on the Richter scale jolted parts of the country today.
The quake was Pakistan's deadliest since the devastating temblor of 2005 that killed some 75,000 in the Kashmir region.
Balochistan comprises about half of Pakistan's territory but is the country's least populated and least developed province.