At least 780 people have died from a heatwave in southern Pakistan, officials say, as medics battled to treat victims after a state of emergency is declared in hospitals.

The majority of the deaths occurred in the port city of Karachi, Pakistan's economic hub of around 20 million people, where temperatures reached 45 degrees Celsius at the weekend.

The heatwave has coincided with severe electricity cuts and the holy month of Ramadan, when most Muslims do not eat or drink during daylight hours.

"The heat wave death toll has reached close to the 800 mark in the last four days," Anwar Kazmi, a senior official of the private charity, the Edhi Foundation, said

"We are planning to expand the Edhi morgue to cope with a situation like this in future."

Pakistan's prime minister Nawaz Sharif called for emergency measures to be put in place as medics battled to treat patients.

The highest number of deaths have been recorded at Karachi's largest hospital, Post Graduate Medical College Hospital, where they have treated more than 3,000 patients, doctor Semi Jamila said.

"More than 200 of them were either received dead or died in hospital," he said.

The deaths came as the overwhelmingly Muslim country of around 200 million people observes the Islamic holy month of Ramadan, during which eating and drinking is forbidden from sunrise to sunset.

Pakistan's largest charity, Edhi Welfare Organisation, said their two morgues in Karachi had received more than 400 corpses.

Electricity shortages have crippled the water supply system in Karachi, hampering the pumping of millions of gallons of water to consumers, the state-run water utility said.

Among the heatstroke victims is Rasheed Ahmed's brother, who fell ill after a power cut left him without air conditioning.

"We live in a poor neighbourhood in a rented house with a sheet roof," Mr Ahmed said. "Because of the high temperature and power failure my brother fell ill and couldn't get medical treatment in time."

"That resulted in his death."

A woman uses cardboard to fan her son while they wait for a medical check-up. ( Reuters: Akhtar Soomro )

Pakistan's meteorological office said temperatures remained around 44.5 Celsius in Karachi on Tuesday but forecast thunderstorms for the evening.

The provincial government, meanwhile, announced a public holiday to encourage residents to stay inside, an official said, but many workers, such as labourers, cannot afford days off, and are continuing to head out into the intense sun while fasting.

Some residents also took to hosing each other down with water to avoid collapsing from heatstroke."

"The biggest problem right now is basically that we are all fasting, like the majority of us, so we all get dehydrated at one point and we cannot obviously drink water," said Sheema Khan, a blogger currently in Pakistan.

Clerics urge people at risk to break fast

Tahir Ashrafi, a prominent Islamic cleric, urged those who were at risk of heatstroke to abstain from fasting.

"We [religious scholars] have highlighted on various television channels that those who are at risk, especially in Karachi where there is a very serious situation, should abstain from fasting," he said.

"Islam has drawn conditions for fasting, it is even mentioned in the holy Koran that patients and travellers who are not able to bear fasting can delay it and people who are weak or old and are at risk of falling sick or even dying because of fasting should abstain."

An official from the National Disaster Management Authority said heatstroke treatment centres would be established at all hospitals across the province to provide "emergency medicines for heatstroke victims".

The deaths come a month after neighbouring India suffered a deadly heatwave with more than 2,000 deaths.

Hundreds of mainly poor people die at the height of summer every year in India, but this year's toll was the second highest in the country's history.

ABC/AFP