The death toll from a heatwave in southern Pakistan rose to 750 on Wednesday, but the scorching weather showed signs of easing, bringing some respite to the sweltering city of Karachi.

Temperatures in the city, which is Pakistan's largest and has seen the majority of the deaths, were forecast to peak at 38 degrees Celsius (100 Fahrenheit), down from the 40-plus highs of recent days.

Winds have shifted to the southwest, blowing cooler air into the port city from the Arabian Sea, and the Pakistani Met Office has predicted rain, which would bring lower temperatures.

Roads in the normally bustling city were largely deserted on Wednesday after the Sindh provincial government declared a public holiday to encourage people to stay indoors out of the sun.

A state of emergency is in force in hospitals which are struggling to cope with the thousands of people affected by heatstroke and dehydration.

The change in weather will come as welcome relief for the economic hub, where maximum temperatures have hovered around 44-45 degrees C since Saturday, though officials warned the death toll may still rise.

"The weather is getting better now and we hope that the people would bear with it now," a senior official at the provincial health ministry said.

"The latest death toll, which we collected late last night was 750."

The National Disaster Management Authority has been setting up dedicated heatstroke treatment centres to try to cope with the volume of patients.

At one centre, van driver Mohammad Sharif lay helpless on a bed, a wet cloth draped on his head while his wife and brother rolled cold water bottles on the soles of his feet to cool him.

Pakistan's largest charity, Edhi Welfare Organisation, said their mortuaries in the city had received more than 600 bodies since Saturday and the sheer volume of corpses arriving meant they were struggling to keep them properly chilled.

Anwar Kazmi, a spokesman for the foundation told AFP the situation was unprecedented in his 40 years working with the charity.

"I have never seen such a big number of deaths because of heatwave," he said.

"I remember some 10 years ago, some 30 people died in the whole summer season but this heat is beyond our comprehension."

- Grave shortage -

Victims' families have also faced challenges in burying their dead, as grave-diggers have struggled to keep up with demand in the scorching heat.

Blasting summer heat is not unusual in Pakistan, and some parts of the country regularly experience higher temperatures than those seen in Karachi this week, without serious loss of life.

But this year's heatwave has coincided with the start of the Islamic fasting month of Ramadan, during which millions of devout Pakistanis abstain from food and drink from sunrise to sunset.

Under Pakistani law, it is illegal for Muslims to eat or drink in public during daylight hours in Ramadan.

The majority of the deaths in Karachi have been among the poor and manual labourers who work outdoors, prompting clerics to urge those at risk of heatstroke not to fast.

Dispatch rider Danish Ali, 26, was among those to abandon his fast in the face of the killer heat.

"I started feeling sick and faint. I skipped today's fasting and hope Allah will forgive me," he told AFP.

The situation has not been helped by power cuts -- a regular feature of life in Pakistan -- which have stopped fans and air conditioners from working.

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.

The Pakistan Peoples Party (PPP), which rules Sindh, announced a protest against the K-Electric power company and the federal government.

The plan drew criticism on social media, with some people saying the PPP should devote its energy to addressing the crisis rather than politicising the issue.

The respected English-language daily Dawn accused both provincial and federal ministers of trying to pass the buck.