Top Edhi volunteers bring a boat to evacuate stranded people; a man carries his child as he wades across a flooded street after heavy monsoon rainfall in Karachi on Monday. AFP

A weather emergency was declared in Karachi on Monday as heavier-than-usual monsoon rains continue to lash Pakistan's biggest city, flooding homes and making streets impassable.

The provincial Sindh government announced a public holiday on Monday in Karachi and Hyderabad in a bid to avert flood chaos, but low-lying areas - already drenched by weeks of heavy rain - were soon the scenes of devastation.

Taking to Twitter, Sindh government spokesperson Murtaza Wahab wrote that apart from Karachi, there will be a public holiday in Hyderabad too because of rains.

"Due to heavy rainfall which is expected to continue even tomorrow, the Sindh government has decided to declare Monday, the 25th of July as a public holiday in Karachi and Hyderabad Divisions,” tweeted Wahab.

"More rains are forecast in Karachi until tomorrow," warned Sardar Sarfraz, director of the Met office.

At least four people have been killed and many others injured in Karachi was rain wreaked havoc in the port city, flooding the residential and commercial areas, killing and injuring people, damaging properties and disrupting the power and telecommunication system. A teenage boy, identified as Zia, was electrocuted in Liaquatabad area. He was taken to the Abbasi Shaheed Hospital.

His family said he received an electric shock from a service mast. In Surjani Town, a man, who is yet to be identified, died of electrocution.

Police said the deceased received an electric shock from the snapped electricity wires. His body is yet to be identified. He died of electrocution. Police said the deceased received an electric shock from the snapped electricity wires. His body was taken to the morgue after completion of legal formalities at the Abbasi Shaheed Hospital.

A 40-year-old man, identified as Shan Hussain, was electrocuted near the Memon Masjid in the limits of Kharadar police station. His body was taken to the Jinnah Postgraduate Medical Center. Meanwhile, a couple and their two kids were injured as the roof of their house collapsed in Korangi.

A couple and their two minor children were wounded after a wall of their house collapsed on them in Baldia Town within the limits of Saeedabad police station.

Police and rescuers reached the property and took the family to the Civil Hospital, Karachi. They were later identified as 35-year-old Nadeem, his wife Pathani, two year-old-son Krishna and daughter Samina, 5.

Meanwhile, the Pakistan Meteorological Department (PMD) said Quaidabad received the heaviest rainfall, measuring 86.5mm. "The intermittent showers are expected to continue till 12pm tomorrow,” forecast the PDM.

The National Disaster Management Authority said at least 312 people had died since June as a result of the monsoon rains.

Pakistan ranks eighth on a list of countries most vulnerable to extreme weather caused by climate change, according to the environment NGO Germwatch.