A heatwave in Pakistan's largest city, Karachi, and other districts of southern Sindh province has killed at least 400 people, health officials have said.

Sindh province's Health Secretary Saeed Mangnejo told AFP news agency that the provincial government had imposed a state of emergency at all hospitals, cancelling leave for doctors and other medical staff and increasing stocks of medical supplies.

The southern port city of Karachi saw temperatures reach as high as 45 degrees Celsius on Saturday, just short of an all-time high of 47C in June 1979.

Dr Seemin Jamali, the head of the emergency department at state-run Jinnah Hospital said more than 100 people had died at the hospital.

"They all died of heat stroke," she said.

A large number of the deceased were old people, Jamali was quoted as saying by the Dawn.com news website said.

Officials said all the deaths had occurred since Saturday evening.

Power outages

Pakistan's Meteorological Department said temperatures would likely subside in the coming days, but doctors have advised avoiding exposure to the sun and wearing light cotton clothes.

The high temperatures were made worse by frequent power outages, sparking protests in several parts of Karachi, a sprawling city of 20 million.

Electricity cuts in turn crippled Karachi's water supply system, hampering the pumping of millions of gallons of water to consumers, the state-run water utility said.

Prime Minister Nawaz Sharif has warned electric supply companies that he would not tolerate power outages during the Muslim fasting month of Ramadan, an official in Sharif's office said.

Karachi University in a statement said that it had postponed its exams for at least one month due to the extreme weather.

Edhi Welfare Organisation, Pakistan's largest charity, said their mortuary had been packed to capacity with heatwave-related deaths and other casualties as 150 bodies were placed there.

"We had to bury some 30 unclaimed bodies to make room in the mortuary," Edhi official Anwar Kazmi told AFP.