At least 21 people have died and 66 others have been hospitalised with exhaustion as soaring temperatures and high humidity hit Cairo and other parts of Egypt, according to the health ministry.
The victims, who all died on Sunday, succumbed as temperatures reached highs of 42C - in conditions made less bearable by elevated humidity levels.
Fifteen people died in Cairo, four in the western province of Marsa Matruh and two in Upper Egypt's Qena province, the ministry said in a statement on Monday.
Those who died, including seven women, were all aged over 60.
Sixty-six people were admitted to hospital after suffering from exhaustion, including 37 who are still under observation, the ministry said.
"There is a big rise in temperature compared with previous years. But the problem is the humidity which is affecting people more," Hossam Abdel Ghaffar, a ministry spokesperson, said.
"Long exposure under the sun is a killer."
Meteorological officials confirmed that temperatures in Cairo and some parts of the country were higher than average.
"The temperature is higher by four to five degrees than what is usually seen, and the humidity is very high this month," Waheed Soudi, head of analysis in the Egyptian Meteorological Authority, told AFP news agency.
On Monday, the temperature touched 37C by late morning, with state media reporting that the heatwave was expected to last until the middle of August.
Although it is not uncommon to see summer temperatures in the high 30s throughout Egypt, it is rare for humidity levels to remain elevated during excessively hot weather.