A passenger ferry capsized off the coast of Tanzania on Wednesday.
Angry Islamists burned churches and clashed with police in Zanzibar to protest against the arrest of a senior member of their movement, police said.
Hundreds of members of the separatist Islamist group Uamsho (Awakening) clashed with police and burned at least two churches during the confrontation in Stone Town, capital of the semi-autonomous Tanzanian archipelago and a Unesco world heritage site.
Up to 200 rioters from the group threw stones at police officers, who responded by firing tear gas and using water cannons in an effort to disperse the crowd and restore order on 27 May.
Witnesses said some of the protesters were armed with machetes and iron bars.
On the preceding day, a group of angry protesters torched two churches, prompting fears that Christians living on the island could be targeted. Zanzibar is inhabited by a majority of Muslims.
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Members of Uamsho also stormed shops and clubs, burning up to three bars and taking cash.
While the group is known by its Swahili acronym Uamsho, its full name is Association for Islamic Mobilisation and Propagation.
The sect wants Zanzibar to end its union with mainland Tanzania and insists on the preservation of Islamic customs.
Shops closed down in the aftermath of the attacks and residents of the secular ruled island remained in their houses for safety.
Police said they made some arrests, but are still hunting for leaders of the group.
"We are searching the group's top leaders and all those involved in riots so that they are taken to court,", Zanzibar police commissioner Mussa Ali said.
"We decided to use tear gas because the rioters were threatening people's lives by torching churches and blocking roads, especial at Michenzani area. We couldn't tolerate them as they were destructing government and people's properties," he added
The group denied the accusations and called for calm, saying in a statement: "The Uamsho association is not involved in any acts of breach of peace. We would like to urge all Muslims and Zanzibaris to continue to maintain peace and tranquillity in the country."
Although the Foreign and Commonwealth Office has not issued any travel restrictions, following the latest surge in violence, a statement on its website warns that tourists should "exercise caution, avoid large demonstrations and political rallies and be alert to any developments which might trigger unrest" as well as monitor the travel advice page and the local media for information.
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