Two bombs exploded minutes apart in the main city on Tanzania's semi-autonomous Zanzibar archipelago yesterday without causing casualties, police and witnesses said, adding to tensions on the islands after a disputed election. Tanzania held national elections last week, which included a vote for local authorities in Zanzibar, traditionally a bastion of opposition to the central government. But Zanzibar's election commission annulled the vote for the island's president, citing "gross violations". The opposition rejected the move, saying it had won that poll. Police said the bombs were "home-made" devices and a mobile phone used to detonate one had been found.