Pakistan has declared heat wave emergency as the death toll topped 450 in southern Sindh province.
The National Disaster Management Authority (NDMA) said it had received orders from PM Nawaz Sharif to undertake immediate response measures.
The army has also been deployed to set up heat stroke centers and assist the NDMA, it added.
Many of the victims are elderly people from low income families.
Hundreds of patients suffering from the effects of the heat wave are being treated at government hospitals, provincial health secretary Saeed Mangnejo said.
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 outages seem to have made matters worse.
Sporadic angry protests have taken place in parts of the city, with some people blaming the government and the city’s main power utility, K-Electric, for failing to avoid deaths.
PM Nawaz Sharif had announced that there will be no electricity cuts but outages have increased since the start of Ramadan.
According to Pakistan’s metrological office cooler weather is forecast from June 23.
The all-time highest temperature reached in Karachi is 47C (117F), recorded in 1979.
Last month, nearly 1,700 people died in a heat wave in neighboring India.