Scores missing as ship voyaging from Zanzibar to Pemba island capsizes off coast of East Africa.

A ferry carrying more than 500 passengers from Zanzibar to Pemba Island off the East African country of Tanzania has capsized and scores of people are missing, police said on Saturday.

About 250 people have been rescued and 40 bodies recovered, the government said.

Zanzibar and Pemba are the two main islands of the Zanzibar archipelago, a popular destination for tourists visiting their pristine Indian Ocean beaches.

"We are fearing the greatest calamity in the history of Zanzibar. This is a disaster," said a government official in Zanzibar, who declined to be named.

Survivor Abdullah Saied said the ferry, MV Spice Islanders, was heavily overloaded and sank in an area with heavy currents in deep sea between mainland Tanzania and Pemba Island at about 01:00 am.

Thousands of residents mobbed the docks of Stone Town on Zanzibar, an island near Pemba, waiting for news, The Associated Press reported.

One man was screaming that he had lost 25 members of his family, including his sisters, his wife and grandsons. He was too upset to give his name. Many of those present expressed anger that the ship had been allowed to leave port so overloaded and called on government officials to resign.

"Some 260 passengers have so far been rescued ... we have recovered several bodies but I can't give you the exact death toll at the moment because the situation is very volatile," Mussa Alli Mussa, Zanzibar's police commissioner stated.

Mussa said rescue workers with the aid of some fishing boats in the area were helping to rescue more passengers and recover bodies.