At least 100 people have died after a ferry sank off the Tanzanian island of Zanzibar with more than 500 people onboard.Many more are missing, but some 260 people, including 60 children, are reported to have been rescued.The MV Spice Islander was travelling between Zanzibar's main island, Unguja, and Pemba, the archipelago's other main island - popular tourist destinations.The ferry was bringing people back from holiday at the end of Ramadan.The authorities are struggling to cope and have asked for foreign help.The Zanzibar government has set up a rescue centre and called up on all reserves to join the rescue effort. It has also called for support from other countries, such as South Africa and Kenya.The survivors were ferried by privately owned fast ferries and brought back to the main harbour in the historic Stone Town, Zanzibar police commissioner Mussar Hamis said.Bodies are being washed up on the shore. So far 100 have been recovered so far, says the BBC's Ali Saleh in Zanzibar.Thousands of relatives are waiting for news of loved ones who may have been on ferry, our reporter says.Mohammed Aboud, Zanzibar's state minister for emergencies, said 259 survivors had been rescued, but 40 of those were seriously injured.Catherine Purvis, a British tourist in Zanzibar who waiting for a ferry to take her to the commercial capital, Dar es Salaam, says she saw lots of bodies being brought out of the water."I'm standing at the port in Zanzibar with about 10 other British and American tourists."Our ferry has been delayed as they're using all ferries to rescue the people from the ship."People are being carried across in front of us on a drip. There are lots of body bags."Holding onLocal helicopter pilot Captain Neels van Eijk flew over the disaster area."We found the survivors holding onto mattresses and fridges and anything that could float. It's hard to tell the exact numbers, but I'd say there were more than 200 survivors in the water and some bodies too," he told the BBC."By then, there were a few boats that had made their way out. They were looking for survivors, but although the sea wasn't so rough, the waves were high so it was difficult for them to spot them."We flew to the boats and guided them to the survivors so that they could pick them up. There were also quite a few bodies in the water."The ferry left Unguja at around 21:00 (19:00 GMT) and is said to have sank at around 01:00 (23:00 GMT).It was heavily overloaded and some passengers refused to board as a result, survivor Abdullah Saied is quoted as saying by the AP news agency.