Nine deaths were reported from Thatta and Tharparkar in the interior of the Sindh province, health officials said.
"Five people died from dehydration and heat stroke in Thatta and four in Tharparkar yesterday," a Sindh health ministry official told PTI.
The total number of deaths in Sindh province has climbed to around 141 since the heatwave started on Friday.
Sindh Chief Minister Syed Qaim Ali Shah told reporters in Karachi yesterday that emergency had been declared in all the government hospitals of Karachi and other cities in the province.
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Temperatures of around 44 to 45 degrees Celsius have been recorded in the port city of Karachi since Friday which was also the first day of the holy month of Ramazan. The scorching heat is expected to continue today as well.
"Most of the mortality cases we have got at our hospital are of elderly people and the homeless due to suffocation, dehydration and heat strokes," said Seemi Jamali, medical superintendent at the Jinnah hospital.
Ahmed Mashadi, a doctor at Abbasi Shaheed hospital, confirmed that 20 people had died due to the heatwave at the hospital while a health official said dead bodies were also brought to other hospitals in the city.
Over 100 patients admitted in the Jinnah hospital are getting treatment for heat-related illnesses, said Jamali, a doctor.
"The relatives bring the bodies to us because of the extreme hot conditions they can't keep the bodies of their dear ones at home for long," said Anwar Kazmi, a spokesman for the Edhi welfare trust and foundation, a charity organisation.
Pakistan's Meteorological Department recorded Saturday as the hottest day of this year's summer in Karachi, where the mercury shot up to 45 degrees Celsius.
Frequent power outages have also compounded people's woes and led to violent protests in several parts of Karachi, Pakistan's largest city with a population of 20 million.