At least 775 people have died of heatstroke, dehydration or other heat-related illnesses in Karachi, the country's largest city, since Saturday, according to government figures.
"The mortuary is overflowing, they are piling bodies one on top of the other," said Dr Seemin Jamali, a senior official at the Jinnah Postgraduate Medical Centre (JPMC), the city's largest government hospital.
"We are doing everything that is humanly possible here," she said, adding that since Saturday, the JPMC had seen more than 8,000 patients with heat-related symptoms. Of those, 384 patients had died, she said.
"Until [Tuesday] night, it was unbelievable. We were getting patients coming into the emergency ward every minute," she said.
Among those who have died, most have been either elderly or poor, officials say.
On Wednesday, the grounds outside the JPMC's Emergency Ward were teeming with people, many seeking relief goods such as blocks of ice, cold water and juices from one of the many aid tents that have sprung up there overnight. Inside the ward, the crowds have begun to lessen.
"Mostly people coming here are facing heatstroke and they are old people. Their ages are around 45 to 50 years old, so the older they are, the more serious problems they are facing," Junaid Ahmad, a volunteer, told the Reuters news agency.
The provincial government in Sindh has declared Wednesday a public holiday for schools and government offices due to the extreme heat, but many private offices have remained open.
A state of emergency has also been declared in the province's hospitals, while the government has also set up several heatstroke aid centres.
"We are launching awareness programmes through radio channels and in hospitals, that people must not expose themselves to direct heat and must drink plenty of water," Jam Mehtab Dahar, the provincial health minister, told Al Jazeera.
Dr Jamali said that one of the reasons why there were so many deaths during this heatwave was that "people have no coping mechanisms, and there are power outages, they don't even have fans in their places", and that many of those who died were those with existing health problems.
Temperatures have hit as high as 43 degrees Celsius since Saturday, accompanied by high humidity and a lull in the city's usually cooling sea breeze.
The maximum temperature recorded in Karachi on Tuesday was 41C, while other cities in the province such as Sukkur, Jacobabad, and Larkana, hit highs of 45C, 44C and 43C respectively.
The spike in temperatures came as many in Muslim-majority Pakistan are fasting during daylight hours to observe Ramadan, further exacerbating the situation.
Repeated power outages across the province, and particularly in Karachi, have also meant that people have not been able to find respite indoors. Some Karachi residents told Al Jazeera they were without electricity for more than 12 hours everyday.
K-Electric, Karachi's power supply and distribution company, says that it is struggling to deal with a spike in demand, as residents have been keeping air conditioners on for longer during the heatwave.
The power outages have also affected the water supply to the city, which was already struggling to meet heightened demand during the summer months.
"There is definitely a shortage of water. The demand is double of what it normally is. The government has taken steps to arrange for 3,000 water tankers to be provided for free in areas where the water shortage exists," said Dahar, the health minister.
"We hope for the best. We are praying to Allah for the temperatures to come down."
While the government estimates that at least 30 people have died in other parts of Sindh, those figures could be far higher, given the lack of access to hospital facilities there.
Many of these areas have seen even higher temperatures and more widespread power outages than Karachi.
Source: Al Jazeera