Separately two men were killed when they were struck by lightning in Badin district of southern Sindh province, of which Karachi is the capital, raising the total death toll in southern Pakistan to 28, provincial health minister Syed Sardar Ahmed told AFP.

Of a total of 26 deaths in Karachi, 18 were caused when the roofs of houses caved in, and eight by electrocution, he said.

"The dead included 15 males and 11 females in Karachi," Ahmed said.

Severe weather had forced the provincial government to declare a state of emergency in Karachi's hospitals, with holidays cancelled for all doctors and paramedics, officials said.

Most of the city was under knee-deep water caused by the heavy rains, causing traffic chaos.

Authorities have closed schools and colleges due to transportation difficulties and hazards including fallen power lines.

Officials said Karachi had received 191mm (7.5 inches) of rain in the previous 48 hours, with chances of light rain in next 24 hours.