Image copyright Reuters Image caption Wednesday was declared a public holiday with people being encouraged to stay indoors

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 have 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 mortuaries had received hundreds of corpses and were now full.

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

Wednesday has been declared a public holiday by the administration in Sindh in the hope that people will stay indoors out of the sun.

On Tuesday as temperatures reached 45C (113F), Pakistan's Prime Minister Nawaz Sharif called for emergency measures, and the army was deployed 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''

Temperatures in Karachi have dropped to 34C (93F) thanks to wind from the sea but there is anger among local residents at the authorities because days of power cuts have restricted the use of air-conditioning units and fans.

Karachi resident Muzzafar Khan told the Associated Press: "The electricity hasn't been working since seven this morning and even during the night there were frequent breakdowns.

"We are forced to sleep in the streets. Ours are small houses; the power supply cables get damaged frequently and nobody is dealing with this situation."

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

Some clerics have issued statements and appeared on television reminding people they are not obliged to fast if they are weak, elderly or simply not fit to fast, the BBC's Shahzeb Jillani says.

How the body copes with heat 37-38C Normal body temperature 39-40C Brain tells muscles to slow down

40-41C Heat exhaustion/heat stroke

42C Body shuts down

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

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

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

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

Hot weather is not unusual during the summer months in Pakistan, but prolonged power cuts seem to have made matters worse, our correspondent 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

Sporadic angry protests have taken place in parts of Karachi, with some people blaming the government and the city'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.

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.

Pakistan heatwave 780 dead in Karachi 30 deaths elsewhere in Sindh province

45C maximum temperature

2,177 died in Indian heathwave

Analysis: Shahzeb Jillani, BBC Karachi reporter

There's anger on the streets 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".

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.

How the body copes with extreme heat

Image copyright Reuters

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 bove 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. If not treated immediately, the 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