Then water rushed through and killed the engines, sending the M.V. Spice Islanders upside down and pitching hundreds of people into the deep sea early Saturday morning. A witness counted nearly 200 bodies, and the president of the nearby island of Zanzibar said more than 570 people were rescued, suggesting the boat was overloaded. Some survivors said the boat's capacity was about 600.
Those lucky enough to find something to cling to floated in the dark waters for at least three hours until the strong currents began to wash them up on the white sandy shores of Zanzibar. As the sun rose, news of the tragedy had already spread throughout the community and thousands of people were on the beach, desperately hoping their family members would be the next to emerge from the waves. One man - too upset to give his name - screamed over and over again the names of 25 missing family members, including his sisters, wife and grandsons.
Throughout the day, police waded through the clear waters to shore, carrying bodies on stretchers, wrapped in brightly colored cloth and blankets. The smallest bundles - the children - they carried in their arms. Tourists on the popular island helped survivors and local charities provided blankets and tea.
It's unclear how many people were killed or how many people were on the boat when it capsized.
A reporter for ITV, a local television station, said he had seen 189 bodies. The president of Zanzibar, Ali Mohammed Shein, said 572 people had been rescued and declared three days of mourning for the disaster. A survivor, Khamis Mohamed, said it was carrying hundreds more than its official capacity of 600.
Others reflected that sentiment - some very vocally.
Dozens of Tanzanians on the island of Zanzibar who spoke to The Associated Press expressed anger that the boat had been allowed to leave port at all and asserted it was overloaded. Residents of Stone Town said they had frequently referred to the vessel as "a disaster waiting to happen." Survivor Abdullah Saied said some passengers had refused to board the boat, saying it was too full, as it left the mainland port of Dar es Salaam for Pemba Island, north of Zanzibar.
At 1 a.m. on Saturday, it sank in an area with heavy currents in deep sea between mainland Tanzania and Pemba Island. Boats frequently traverse the route, but travel times vary depending on the vessel.
Passengers described the terrifying moment when they realized something had gone wrong. People began to scream as the boat tilted to one side and water rushed in.
"I realized something strange on the movement of the ship. It was like zigzag or dizziness," said 15-year-old Yahya Hussein, who survived by clinging to a plank of wood with three others. "After I noticed that I jumped to the rear side of ship and few minutes later the ship went lopsided."
Hussein said there had been many children aboard the ship. After the ship began to list, water rushed through the main cabin and stopped the engines, said Mwita Massoud, another survivor. People began to jump into the sea.
The green and hilly island of Pemba is often described as one of the best scuba diving destinations in the world. Tanzania is heavily reliant on tourism to support its economy.
In the hours after the sinking, the government strongly discouraged journalists from reporting the event and refused to give journalists information.
"We appeal for calm to the public. The government is doing its best it can to handle the situation. There is no need to panic," said Mohammed Aboud Mohammed, the minister for state in the vice president's office.
In 2006, another ship capsized in Zanzibar, killing hundreds. But the angry residents of Stone Town who spoke to the AP said the government still did not invest in better ferries or boats capable of mounting a rescue.
It was unclear if any foreigners were aboard.
Britain's Minister for Africa Henry Bellingham extended his "deepest sympathy" on behalf of the British government to those affected.
"I was deeply saddened by news of the ferry disaster off Tanzania earlier today which claimed the lives of so many people," he said in a statement. "... At this difficult time, our thoughts are with the people of Tanzania as they come to terms with this terrible tragedy."
Associated Press writers Sukhdev Chhatbar in Arusha, Tanzania and Katharine Houreld in Nairobi, Kenya contributed to this report.