Climate change will be the biggest threat to global health in the next 50 years, the study concluded, as gains in development could be offset by adverse weather conditions. Poor infrastructure in developing countries contributes to the threat, according to Costello.

The heat wave in Karachi coincides with major power cuts, leading to harsh criticism of the provincial government and K-electric, the company that supplies electricity to Karachi — the country's richest city and home to 20 million people.

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.

Another facility, Civil Hospital, also had a large number of patients suffering from heat stroke. “A few old fans blew 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,” Reuters news agency reported from the scene.

"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 111 degrees Fahrenheit in the steamy port city in recent days, up from a normal summer temperature of 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.

A morgue run by the charity Edhi Foundation had received more than 400 people who died of complications from the heat, official Anwar Kazmi told Reuters.

“Generally speaking, the reports you get grossly underestimate the number of deaths in the immediate term,” Costello said. “I would not be at all surprised if the figure ended up 10 or 20 times higher.”

The paramilitary Rangers force has set up medical camps at several points in the city where they are providing water and anti-dehydration salts.

Opposition parties are increasingly criticizing the provincial government over its handling of the crisis, which has been exacerbated by severe power cuts.

One lawmaker suggested that the parliament should turn off its air-conditioning for an hour to show solidarity with the people.

The electricity grid has been overwhelmed as people switch on fans and air conditioners — and as families all begin to cook at the same time. The holy month of Ramadan, when most Muslims do not eat or drink during daylight hours, began on Friday. As families across the country break their fast at sunset, they put a great deal of stress on the city's infrastructure.

Al Jazeera and wire services