Pakistan's prime minister has called for emergency measures as the death toll from a heatwave in southern Sindh province reaches nearly 700.
The National Disaster Management Authority (NDMA) said it had received orders from Nawaz Sharif to undertake immediate response measures.
The army has also been deployed to set up heat stroke centres and assist the NDMA, it added.
Many of the victims are elderly people from low-income families.
The death toll rose so dramatically because many of the recent fatalities were already in a critical condition and had been struggling for days.
Health officials said many deaths have been in the largest city, Karachi, which has experienced temperatures as high as 45C (113F) in recent days.
Sindh province Health Secretary Saeed Mangnejo said that 612 had died in the city's main government-run hospitals during the last four days. Another 80 are reported to have died in private hospitals.
The demand for electricity for air conditioning has coincided with increased power needs over Ramadan, when Muslims fast during daylight hours.
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 the city, with some people blaming the government and Karachi's main power utility, K-Electric, for failing to avoid deaths, the report adds.
The prime minister had announced that there would be no electricity cuts but outages have increased since the start of Ramadan, he reports.
Pakistani newspapers came down heavily on the authorities for their handling of the crisis yesterday.
"The blame is squarely on the shoulders of the government for its lacklustre performance in providing water and electricity," 'The Nation' said bluntly, with the 'Daily Times' agreeing that the prime minister now needed to take "some bold decisions". Emergency services have set up medical camps in the streets to cope with the soaring number of casualties.
One of Karachi's biggest public hospitals said all its beds were full, with more than 200 people dying there of dehydration or heat exhaustion.
"Some were brought in dead, while others died during treatment," said Dr Seemin Jamali, joint director at Jinnah Postgraduate Medical Centre.
Civil Hospital was also full of heatstroke patients. A few old fans blew the sweltering air past stray cats sprawled in the dark corridors as friends of an unconscious policeman rushed outside to buy him the cold water the hospital could not provide.
"This is how it is. No one cares for common poor man here," Khadim Ali complained as he fanned his cousin, Shahad Ali, a 40-year-old vegetable vendor who collapsed in the heat.
Temperatures have touched 44 degrees Celsius (111 degrees Fahrenheit) in the steamy port city in recent days, up from a normal summer temperature of 37C (99F). But rain is due.
"A sea breeze will set in some time tonight. The temperature will come down as the monsoon rain enters the Sindh coast, bringing rain to the city," said Ghulam Rasool, director general of the Meteorological Department.
Meanwhile, a morgue run by the charity Edhi Foundation had received more than 400 people who died of complications from the heat, official Anwar Kazmi said yesterday.
The paramilitary Rangers force has set up medical camps at several points in the city - the country's richest and home to 20 million people - where they are providing water and anti-dehydration salts.