Swarms of flies are descending on Karachi in what residents say are record numbers this rainy season, adding to the misery of the city's monsoon “hell”.

Heavy rains have inundated the sprawling port city of nearly 20 million people for weeks, overwhelming shoddy drainage systems clogged with mountains of uncollected garbage and flooding neighbourhoods with raw sewage.

“I have never seen such a fierce presence of flies in my life,” Karachi resident Abdul Aziz, 45, told AFP.

“Clouds of flies keep covering the food at the market. It's repulsive — they cover the fruit so much that you can't see beneath them.”

At a market in Surjani town, meat trader Zahid Ali looked on as flies engulfed the area.

“If the customers come, they impulsively leave after seeing the swarms of flies,” said Ali, adding that an increasing number of people working in the market had fallen ill.

Shershah Syed, a health rights activist and prominent surgeon in Karachi, said many illnesses were on the rise because of flies and mosquitoes.

“This time (the flies are) the worst ever as rain water is unable to drain and the garbage heaps are not dealt with,” Syed said.

“The number of children entering hospitals for diarrhea or dysentery has jumped several fold this year. The number of children — who are the most vulnerable to the fly-borne diseases — has increased by about 10 times.”

While Karachi is responsible for 60 per cent of Pakistan's economic output, the city has long endured creaky infrastructure, illegal construction and failing municipal services.

This week, the Economist Intelligence Unit ranked Karachi as one of the least liveable cities in the world along with the likes of war-torn Libya's Tripoli and the crisis-hit Venezuelan capital Caracas.

“People in Karachi are numb to the idea of living with medical waste, overflowing gutters, broken down roads, and a complete lack of any kind of respectable public transport system,” wrote Saadat Ali Zia on Twitter.

“We live in hell,” tweeted Farooq Afridi.