A boy (6) saved his 18-month-old brother from drowning, after a ferry sank off Tanzania's coast, by holding on to him in the sea for four hours, a charity has revealed.

More than 240 people were killed when the ferry went down and some 600 have been rescued - figures indicating that the boat was filled beyond capacity.

Rescue boats, helicopters and divers searched for any remaining survivors yesterday but hopes were fading fast of finding anyone alive in the submerged wreck.

The accident was the worst maritime disaster in the history of Zanzibar, Tanzania's semi-autonomous archipelago and a popular tourist destination.

Most survivors drifted ashore clinging to foam mattresses or wooden planks from the ferry. Some were plucked from the water by a flotilla of pleasure craft, wooden fishing dhows and yachts which set off from the beach to search for survivors.

International charity, Save the Children, said it launched an emergency response for injured and traumatised children.

The charity described the "incredible bravery" of young survivors, including one six-year-old with a lifejacket who saved his 18-month-old sibling by clinging on to him for four hours.

The charity said 79 out of 129 children it has cared for have been reunited with their families.

"Children arrived at our centre freezing, dehydrated and suffering from shock," said Mubarak Maman, the charity's team leader in Zanzibar.

The MV Spice Islander began its voyage in Tanzania's commercial capital, Dar es Salaam, where it was loaded with passengers, and cargo. When it reached Zanzibar it took on more passengers for the trip to the smaller island of Pemba. Police Commissioner Mussa Alli Mussa said: "According to what we've heard so far from survivors, the ship was overloaded with cargo from Dar es Salaam, which included several vehicles, cement and iron rods."

Some passengers got off, when the boat began to list while still in the port.

Many residents angrily asked why the boat had been allowed to leave port so overloaded.

In 1996, a Tanzanian ferry sank on Lake Victoria with as many as 1,000 aboard. Only 114 survived. The Government charged the captain and eight crew with the murder of 615 people.