At least 24 people perished and many more remain missing after a ferry sank near Zanzibar on Wednesday, the second such deadly disaster in the Indian Ocean archipelago in less than a year.

At least two of the dead were Europeans, officials said.

The vessel, which was officially carrying almost 290 passengers and crew, including more than 30 children, went down in choppy waters near Zanzibar after leaving Tanzania's commercial capital Dar-es-Salaam around midday.

Weeping relatives gathered on the quaysides in both Dar-es-Salaam and Zanzibar, where survivors emerged soaking wet from rescue boats.

"We have so far received 24 bodies, including two Europeans," Zanzibar's transport minister Hamad Masoud Hamad told journalists gathered at the main hospital after the sinking of the MV Kalama.

"The rescue operations are continuing... 124 people have already been found alive and we hope that others will be saved," Tanzania's Interior Minister Emmanuel Nchimbi said on national television.

The archipelago is famed both for its white-sand beach resorts and for Stone Town, the old quarter of Zanzibar, which is a UNESCO heritage site, and is a popular tourist destination.

An AFP journalist at the port on the main island said he had seen 55 survivors on the quayside. Medical personnel were handing out blankets as they emerged from rescue boats, before being sent to a nearby hospital.

Some unconscious victims were being carried on stretchers.

Saidi Shabaani, an official in the second vice president's office, said the ferry was carrying 251 adults, 31 children and six crew, according to the passenger list.

The number of foreigners on board was not yet known.

Ferries in the region very often carry additional passengers who do not feature on the official manifest.

The families of the dead and missing were milling around on the waterfront wailing, the AFP reporter said. Crowds were trying to surge past police barricades to get closer to the water.

"This is another tragedy we are investigating. A rescue team from the police and navy have rushed to the scene," Shaabani told AFP, adding that the ferry capsized at around 12:30 pm (0930 GMT).

"We are asking for peace and calm among Zanzibaris and ask them to have faith that the government is doing all it can in these times," Zanzibar's vice president Seif Ali Idd told journalists on the quayside.

More than 200 people perished in September when the ferry Spice Islander capsized while sailing between two of the three main islands that make up Zanzibar, a semi-autonomous part of Tanzania, in one of the worst maritime disasters in Africa in the past decade.

The September disaster is believed to have been caused by overloading, with some angry survivors accusing port and ferry officials of having ignored the protests of passengers that the boat was overcrowded. Over 600 people survived.

According to a statement from Tanzania's Surface and Marine Transport Regulatory Authority (SUMATRA) the MV Kalama left Dar es Salaam around midday and had issued an alert signal after reaching waters off Chumbe Island.

An employee at the eco-lodge on Chumbe, however, said the vessel appeared to have actually capsized off another small island close by, Kwale and that rescue teams had sailed past Chumbe.

The Washington State Department of Transportation said it had sold the MV Kalama and another ferry in 2011 to a Canadian company which operates routes between the African continent and Zanzibar.

Both were built in 1989.