At least 187 people have died after an overloaded ferry sank off the Tanzanian island of Zanzibar with at least 800 people on board.

A government spokesman said 620 survivors had been rescued. Three days of mourning have been declared.

The MV Spice Islander was travelling between Zanzibar's main island, Unguja, and Pemba, the archipelago's other main island - popular tourist destinations.

It is thought to have capsized after losing engine power.

Rescue efforts were hampered by the fact that the overloaded boat had capsized at night.

The ship had been bringing people back from holiday after Ramadan, and had reportedly stopped earlier in Tanzania's commercial capital, Dar es Salaam.

The authorities are struggling to cope and have asked for foreign help.

The Zanzibar government has set up a rescue centre and called upon all reserves to join the rescue effort. It has also called for support from other countries, such as South Africa and Kenya.

Washed ashore

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.

"We are still receiving many bodies by truck loads," Dr Karim Zah of the Mnazi Mmoja Hospital in Zanzibar told Reuters news agency.

"The death toll will likely be much higher."

Dozens of soldiers carrying bodies to shore dotted the white sand beaches at the northern tip of Zanzibar island where thousands of people anxiously awaited news of survivors, the agency adds.

Catherine Purvis, a British tourist in Zanzibar who was waiting for a ferry to take her to 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," she said.

"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."

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