The death toll from a petrol station explosion and heavy flooding in Ghana’s capital, Accra, has risen to more than 150, rescue workers have said.

President John Dramani Mahama described the loss of life as “catastrophic and almost unprecedented” and announced that three days of national mourning would be held next week.

A fire broke out late on Wednesday at a filling station where people had sought shelter from heavy rains that caused widespread flooding. The victims were burned beyond recognition where they stood under the station’s awning, or trapped and incinerated in the wreckage of cars and minivans on the station’s forecourt. Neighbouring buildings were also set ablaze.

Francis Obeng, disaster management coordinator of the Red Cross in Ghana, told Agence France-Presse on Friday: “As of yesterday we have recovered more than 150 corpses. Sixty people have been taken to the hospital.”

He said rescuers, including dozens of volunteers, were assessing the situation and providing relief supplies to people affected. “We are providing blankets, mats and drugs to residents in the seven communities affected by the fire and flooding. UNDP (United Nations Development Programme) officials are also joining us in our efforts to bring succour to those affected.”

Visiting the scene, Mahama said he was heartbroken by the loss of life and blamed the floods partly on people building homes and businesses on the city’s waterways, blocking drainage systems. “We must sit down and strategise to make sure this doesn’t happen again,” he said, pledging £9m to help flood victims.

The opposition leader, Nana Akufo Addo, also went to the site and called it a “dark day in the history of the city”.

Witnesses said the main victims of the blast were low-wage workers struggling home through a seasonal storm that had forced the closure of roads and stoppage of minibus taxis. One woman said she sought refuge at the station from floods that prevented her returning home from work, but soon became alarmed by an overpowering smell of gasoline and moved to stand next to an adjacent bank building.

A power cut plunged the area into darkness and when a generator was switched on at the petrol station to provide light, people cheered at first, Comfort Arhin, a domestic worker, told Reuters. “We heard a pop. Then the fire was walking where the petrol was. Everybody was running. You had to fight for yourself. My bag fell inside the water … All the people in the filling station died.”

Other people died in flooding that left thousands homeless, officials said. The tragedies have highlighted flaws in Accra’s infrastructure, which has failed to keep pace with population growth after years of rapid economic expansion.

Police spokesman Arthur Cephas said the situation in Accra was returning to normal. “Police are protecting lives and properties as well as ensuring that evidence that would assist in the investigation is not tampered with,” he told AFP.

“We have commenced investigations into this monumental national tragedy … more bodies have been recovered from across Accra following the flooding.”