With thousands of victims of heatstroke arriving at over-stretched hospitals, and morgues running out of space, the government of Sindh declared on Wednesday a public holiday to allow people to shelter in the shade where possible.
While the wealthier of Karachi’s 20 million residents have used private generators or gathered in air conditioned shopping malls to avoid the high temperatures, extensive power cuts have left many others without working fridges, fans or water.
Doctors said many deaths have been caused by dehydration.
With the civilian government failing to stem the crisis, the Pakistan military has stepped in, opening 22 health centres to distribute aid.
The director general of the National Disaster Management Authority, Major General Asghar Nawaz, said 800 people had died in Karachi and a further 38 in other parts of Sindh province. However that figure was changing fast, he said.
As the crisis escalated, politicians blamed each other for the failure of infrastructure projects intended to improve power and water supplies.
Local power company K-electric says federal and provincial governments owe it $1bn in outstanding bills.
As bodies piled up, charities said they were planning to expand their morgue facilities, to cope better with such situations in future.
After almost a week of extreme temperatures a sea breeze cooled the city slightly on Wednesday. Monsoon rains are not due for another week.
The average temperature in June is 33 Celsius, but during the heatwave have been around 45 C for the last five days.