The death toll from Pakistan's killer heatwave rose past 1,000 on Thursday, with more fatalities expected, as cloud cover and lower temperatures brought some relief to the worst-hit city Karachi.

Mortuaries and gravediggers in Karachi, Pakistan's largest city and economic hub, have struggled to keep up with the flow of bodies since the scorching temperatures began last weekend.

Hospitals have been on a crisis footing and dedicated heatstroke treatment centres have been set up around the city to treat the tens of thousands affected by heatstroke and dehydration.

"The death toll is more than 1,000 and it may reach up to 1,500," Anwar Kazmi, a spokesman for the Edhi Foundation, Pakistan's largest welfare charity and a leading provider of emergency medical care in Karachi, told AFP.

According to figures collected by AFP from hospitals around the city, a total of 1,079 people have died, though the pace of the deaths has slowed as the weather has cooled in the last two days.

Karachi hospitals have treated nearly 80,000 people for the effects of heatstroke and dehydration, according to medical officials.

After days of temperatures hovering at highs in the mid-40s Celsius (around 110 Fahrenheit), sea breezes and cloud cover have brought some respite to the port city in the last two days.

The Met Office forecast temperatures of around 34 degrees Celsius on Thursday, with 75 percent cloud cover.

"We are now getting fewer and fewer patients and hope the situation will become better with the passage of time," doctor Seemin Jamali from Jinnah Post Graduate Medical College told AFP.

- Gravediggers cash in -

Victims' families have also faced challenges in burying their dead, as some unscrupulous grave-diggers were demanding 50,000 rupees ($500) for a grave -- nearly 10 times the official rate.

City authorities have dug more than 300 graves in two cemeteries to ease the problem, charging relatives the usual 5,800 rupee fee.

Temperatures of 45 C and higher are not uncommon in parts of inland Pakistan, but Karachi normally remains cooler thanks to its coastal location.

This week, however, the cooling breeze that usually blows in off the Arabian Sea was absent.

The city of 20 million inhabitants is a sprawling metropolis with few green areas, poorly adapted to manage intensely hot weather.

Vast areas of concrete absorb heat during the day and radiate it back at night in what climatologists call the "urban heat island" effect.

This year's heatwave has also 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, though the crisis prompted some clerics to advise people they should stop fasting if their health is at risk.

- Education needed -

The majority of the deaths in Karachi have been among the elderly, the poor and outdoor manual labourers, many of whom are paid by the day and may be reluctant to stop work as it would mean losing income.

Doctor Qaiser Sajjad of the Pakistan Medical Association in Karachi said that a lack of understanding of heatstroke among the public -- how to spot symptoms and treat them -- had contributed to the deaths.

"This high mortality in the past five days came mainly because of our failure to educate people," he told AFP.

"We see a blitz of TV advertisements after each and every ball delivered in cricket, similarly we should publicise the need for hydration and other safety measures."

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 and interrupted Karachi's water supply.

The crisis comes a month after neighbouring India suffered its own deadly heatwave which killed more than 2,000 people.