Image copyright AFP Image caption Matters have been made worse by power cuts, angering residents

The death toll from a heatwave in Pakistan's southern Sindh province has passed 800, hospital officials say, as mortuaries reached capacity.

At least 780 people had died in Karachi, BBC Urdu reported. Another 30 deaths were reported elsewhere in the province, state owned PTV said.

The Edhi Welfare Organisation told AFP that their morgues had received hundreds of corpses and were now full.

Officials have been criticised for not doing enough to tackle the crisis.

"More than 400 dead bodies have so far been received in our two mortuaries in past three days," Edhi spokesperson Anwar Kazmi told AFP. "The mortuaries have reached capacity."

On Tuesday as temperatures reached 45C (113F), Pakistan's PM Nawaz Sharif called for emergency measures and the army was deploying to help set up heat stroke centres.

Media playback is unsupported on your device Media caption One labourer told the BBC: ''It's so hot that I can barely speak''

There is anger among local residents at the authorities because power cuts have restricted the use of air-conditioning units and fans, correspondents say.

Matters have been made worse by the widespread abstention from water during daylight hours during the fasting month of Ramadan.

On Tuesday, the National Disaster Management Authority (NDMA) said it had received orders from Mr Sharif to take immediate action to tackle the crisis.

This came as Sindh province Health Secretary Saeed Mangnejo said 612 people had died in the main government-run hospitals in the city of Karachi during the past four days. Another 80 are reported to have died in private hospitals.

Image copyright AP Image caption Thousands of people are being treated in the Sindh province, and some of them are in serious condition

Many of the victims are elderly people from low-income families.

Thousands more people are being treated, and some of them are in serious condition.

Hot weather is not unusual during summer months in Pakistan, but prolonged power cuts seem to have made matters worse, the BBC's Shahzeb Jillani reports.

Sporadic angry protests have taken place in parts of Karachi, with some people blaming the government and Karachi's main power utility, K-Electric, for failing to avoid deaths, our correspondent adds.

The prime minister had announced that there would be no electricity cuts but outages have increased since the start of Ramadan, he reports.

Media playback is unsupported on your device Media caption Dr Seemi Jamali, director of Jinnah Hospital in Karachi, says her team is under "tremendous pressure" treating heatstroke patients

Analysis: Shahzeb Jillani, BBC Karachi reporter

There's anger on the street about the government's slow response to the crisis. The provincial PPP government appeared aloof and unresponsive. The federal government of Prime Minister Nawaz Sharif woke up to the tragic deaths on the third day.

While politicians blamed each other for not doing enough, the army - always keen to seize opportunities to demonstrate its soft power - sprang into action to set up "heat stroke relief camps".

By the fourth day, a campaign was launched to reiterate steps people should take in sizzling temperatures.

Many in Karachi feel that had the authorities moved proactively many lives could have been saved.

The hope now is that with the expected pre-monsoon rains later in the week the weather will improve. That will certainly provide much-needed respite to millions affected by the heatwave, but it won't change the chronic underlying problems this ever-growing city of 20 million faces - a dysfunctional infrastructure and poor governance.

Image copyright Reuters

How the body copes with extreme heat

The body's normal core temperature is 37-38C.

If it heats up to 39-40C, the brain tells the muscles to slow down and fatigue sets in. At 40-41C heat exhaustion is likely - and above 41C the body starts to shut down.

Chemical processes start to be affected, the cells inside the body deteriorate and there is a risk of multiple organ failure.

The body cannot even sweat at this point because blood flow to the skin stops, making it feel cold and clammy.

Heatstroke - which can occur at any temperature over 40C - requires professional medical help and if not treated immediately, chances of survival can be slim.

There are a number of things people can do to help themselves. These include:

Drinking fluids

wearing damp clothes which will help lower the body's temperature

sticking one's hands in cold water

placing fans next to windows as this will draw air from outside, which should be cooler

wearing looser clothes

having a lukewarm shower rather than a cold one

fanning the face rather than other parts of the body

What happens to the body in extreme heat?

Eight low-tech ways to keep cool in a heatwave

Karachi resident Iqbal told the BBC on Monday that no-one in his family could go outside to work because of the temperature and that everyone in their area preferred to stay at home.

"In our area, there is no electricity [since the] morning. We have complained several times, but there is no response from K-Electric," he said.

Image copyright EPA Image caption Curtains to protect against the heat are in great demand

According to Pakistan's metrological office cooler weather is forecast from Tuesday.

The all-time highest temperature reached in Karachi is 47C, recorded in 1979.

Last month, nearly 1,700 people died in a heatwave in neighbouring India.