ISLAMABAD: The meteorological department has issued a warning that more thundershowers are expected to hit Karachi on Sunday, after heavy rains wreaked havoc and brought activity in Pakistan's commercial capital to a standstill.

Karachi was lashed by heavy rains that began on Thursday and continued through Friday afternoon, causing widespread flooding as downpours shattered 89-year-old records. Streets and homes were flooded with sewage water as Karachi's drainage systems are outdated. Dozens of people were killed and many remain missing. 

"Heavy rainfall may further aggravate existing urban flooding and water logging in Karachi," the Pakistan Meteorological Department said in a statement on Saturday, advising all concerned authorities to remain on alert as torrential rains are expected to hit Sindh province again from Saturday night to Monday. 

Sindh Chief Minister Murad Ali Shah told reporters on Friday afternoon that 47 people had died in Karachi in rain-related incidents.

On Saturday, Sindh declared 20 districts, including all districts in Karachi, as calamity affected under the Sindh Calamities Act of 1958, provincial government spokesman Murtaza Wahab said in a Twitter post. The announcement means that residents will be eligible to receive government compensation.

Wahab added in another post that the chief minister had directed electric supply company K-Electric to expedite restoration of electricity. Twitter users reported power outages of over 50 hours.

Every year, many cities in Pakistan struggle to cope with rains during the monsoon season which runs from July through September. Although rains have lashed many parts of Pakistan, Karachi, a city of over 15 million people, has been the hardest-hit. According to Met data which go back to 1931, it has never rained so much in the month of August.

The Met department said Karachi received 223.5 mm of rain in just 12 hours on Thursday alone, the highest amount of rain ever recorded in a single day in the city. The previous 24-hour record was on July 26, 1967, when 211.3 mm was recorded at Masroor base.