Many of the victims are elderly people from low-income families.

Thousands more people are being treated, and some of them are in serious condition.

Hot weather is not unusual during summer months in Pakistan, but prolonged power cuts seem to have made matters worse, the BBC’s Shahzeb Jillani reports.

Sporadic angry protests have taken place in parts of Karachi, with some people blaming the government and Karachi’s main power utility, K-Electric, for failing to avoid deaths, our correspondent adds.

The prime minister had announced that there would be no electricity cuts but outages have increased since the start of Ramadan, he reports.

Analysis: BBC’s Shahzeb Jillani in Karachi

There’s anger on the street about the government’s slow response to the crisis. The provincial PPP government appeared aloof and unresponsive. The federal government of Prime Minister Nawaz Sharif woke up to the tragic deaths on the third day.

While politicians blamed each other for not doing enough, the army – always keen to seize opportunities to demonstrate its soft power – sprang into action to set up “heat stroke relief camps”.

By the fourth day, a campaign was launched to reiterate steps people should take in sizzling temperatures.

Many in Karachi feel that had the authorities moved proactively many lives could have been saved.

The hope now is that with the expected pre-monsoon rains later in the week the weather will improve. That will certainly provide much-needed respite to millions affected by the heatwave, but it won’t change the chronic underlying problems this ever-growing city of 20 million faces – a dysfunctional infrastructure and poor governance.

How the body copes with extreme heat

The body’s normal core temperature is 37-38C.

If it heats up to 39-40C, the brain tells the muscles to slow down and fatigue sets in. At 40-41C heat exhaustion is likely – and above 41C the body starts to shut down.

Chemical processes start to be affected, the cells inside the body deteriorate and there is a risk of multiple organ failure.

The body cannot even sweat at this point because blood flow to the skin stops, making it feel cold and clammy.

Heatstroke – which can occur at any temperature over 40C – requires professional medical help and if not treated immediately, chances of survival can be slim.

There are a number of things people can do to help themselves. These include:

• wearing damp clothes which will help lower the body’s temperature
• sticking one’s hands in cold water
• placing fans next to windows as this will draw air from outside, which should be cooler
• wearing looser clothes
• having a lukewarm shower rather than a cold one
• fanning the face rather than other parts of the body

What happens to the body in extreme heat?

Eight low-tech ways to keep cool in a heatwave

Karachi resident Iqbal told the BBC on Monday that no-one in his family could go outside to work because of the temperature and that everyone in their area preferred to stay at home.

“In our area, there is no electricity [since the] morning. We have complained several times, but there is no response from K-Electric,” he said.

According to Pakistan’s metrological office cooler weather is forecast from Tuesday.

The all-time highest temperature reached in Karachi is 47C, recorded in 1979.

Last month, nearly 1,700 people died in a heatwave in neighbouring India.