At least 20 people drowned on Saturday when a bus travelling to a wedding in central Kenya was swept off a bridge into a flooded river, police said.

"The search and rescue is ongoing but so far 20 bodies have been taken out," said Joseph Yakan, police commander in the Mwingi East area.

He said about 10 people had been rescued but it was unclear how many were aboard the bus when it tipped into the swollen Enziu River about 200 kilometres (125 miles) east of the capital Nairobi.

The bus was carrying members of a church choir and villagers to a wedding.

Witnesses said the driver was trying to cross fast-flowing waters passing over the bridge but the force was too strong, sweeping the bus beneath the churning currents.