Thirteen people died when a wall collapsed on Wednesday, while five people, including two women, were killed elsewhere in the city.

Abdul Wahid, a local resident, described the scene where the 13 died as "complete chaos" when the storm hit.

"It rained heavily, there were strong winds, so these people were taking shelter by the walls to save themselves and then the wall collapsed on them," he told AFP.

Another local resident, Muhammad Sufiyan, gave a similar account. "They were waiting for the wind to calm down so they could cross... and go back to their homes," he said.

"The wall fell on top of them and there was a lot of damage... Some animals were also killed."

Light to moderate rain fell across Sindh province, of which Karachi is the capital, but was heavier in some areas, meteorologists said.

Rain in March is common in Punjab province in east-central Pakistan but unusual in Sindh, which is in the southeast.

"This kind of extreme weather event hasn't happened for a long time," Pakistan Meteorological Department's senior official Ameer Hyder Laghari told AFP.

Pakistan, where 45 percent of people live below the poverty line, is among the countries most vulnerable to climate change, with limited resources dedicated to adaptation.

While South Asia's seasonal monsoon brings rainfall that farmers depend on, climate change is making the phenomenon more erratic.

Last year, monsoon rains in Pakistan killed more than 1,000 people.