One person has been killed and several others injured in a bombing on Tanzania's Indian Ocean island of Zanzibar, police has said.

Police said the bomb went off in the Daranjani business district of Stone Town, the UNESCO-listed historical centre of the capital of the semi-autonomous Tanzanian archipelago, on Friday.

Witnesses said the casualties included worshippers who were coming out of evening prayers from a nearby mosque.

"We are investigating to find out the type of explosive, the criminals and motive. We ask people to help provide information," senior police official Mkadam Khamis told the AFP news agency.

The island is currently hosting a religious gathering of Muslims from across the east Africa region.

The mainly Muslim island, famed for its pristine white-sand beaches, attracts thousands of tourists each year.

In the last few years there have been a number of attacks in Zanzibar, targeting Christian leaders, churches and tourists.

In February, a bomb went off next to an Anglican cathedral near Stone Town, while another at about the same time exploded at a restaurant in the city, without causing any casualties.

Many Muslims living along Tanzania's coast feel marginalised by the secular government, and Tanzanian President Jakaya Kikwete has warned that religious tensions threaten peace in the country.

Zanzibar maintains a political union with Tanzania, but has its own parliament and president.