At least 18 people were killed when a large tree fell into the pool they were swimming in at the base of a waterfall in Ghana , crushing and drowning people enjoying a day out at a popular beauty spot, police said.

The waterfall , near the town of Kintampo, was busy with weekend daytrippers, many of them high school students, when the freak accident happened on Sunday.

A further 20 people were being treated in hospital, Desmond Owusu Boampong, a local police commander, told Reuters. Another police commander said 22 were injured.

"It was the upper part of one of the biggest trees that came crashing down on them. It was a horrifying scene as the area was engulfed in screams and shouts for help as we arrived," fireman Kwaku Boateng told Reuters.

"It was a difficult operation because ... some untrained members of the public joined in a desperate attempt to help. We swung into action immediately with many of our men diving into the water. We used chainsaws and other cutting tools to chop the tree in pieces in order to free the victims," he said.

Pictures used by local media showed thick branches lying in a pool at the base of the waterfall , which drops in large steps over rocks. Other photographs showed bodies, some wearing bathing suits, placed in rows on the ground or piled into pick-up trucks.

The disaster occurred not long after a rainstorm in the area located in Ghana 's middle belt Brong Ahafo region which is known for heavy rains at this time of year.

Ghana 's Tourism Minister Catherine Afeku, who is heading to the scene to meet the families of victims, said an investigation would be conducted. President Nana Akufo-Addo sent his condolences via his official Twitter feed.