Nearly 700 people have died in a severe three-day heatwave in Pakistan, officials said Tuesday, with medics battling to treat patients as a state of emergency was declared in hospitals.

The majority of people died in government-run hospitals in the port city of Karachi, Pakistan's economic hub of around 20 million people, as temperatures reached 45 degrees Celsius (111 Fahrenheit) at the weekend.

"The number of people who have died in the heatwave has now reached 692," said Saeed Mangnejo, a senior provincial health official, adding that the toll may rise futher.

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 told AFP.

Pakistan's largest charity, Edhi Welfare Organisation, said their two morgues in the city had received more than 400 corpses in the last three days.

"The mortuaries have reached capacity," Edhi spokesman Anwar Kazmi told AFP.

Meanwhile, seven people have died from the heat in Punjab province over the past 24 hours, officials 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.

Some clerics have issued public warnings saying that physically weak people can abstain from fasting in these tough conditions.

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.

Temperatures remained at around 44.5 Celsius in Karachi on Tuesday but thunderstorms were forecast for the evening, Pakistan's Meteorological Office said.

"Due to a low depression developing in the Arabian sea, thunderstorms will likely begin this evening and might continue for the next three days," a Meteorological official told AFP.

The provincial government meanwhile announced a public holiday to encourage residents to stay inside, an official said. Many of the victims have been labourers who toil outdoors.

Some residents also took to hosing each other down with water on Tuesday to avoid collapsing from heat stroke.

Prime Minister Nawaz Sharif has issued special instructions to the National Disaster Management Authority (NDMA) and other relevant organisations to arrange urgent assistance for the heatwave victims.

An official from the National Disaster Management Authority told AFP heatstroke treatment centres would be established at all hospitals across Sindh province, of which Karachi is the capital.

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.