A ferry carrying 459 people, mostly high school students, has sunk off South Korea, leaving at least four people dead and nearly 300 missing.
The high number of people unaccounted for - likely to be trapped in the ship or floating in the ocean - raised fears that the death toll could rise drastically, making it one of South Korea's biggest ferry disasters since 1993 when 292 people died.
Local television stations broadcast live pictures of the ship, Sewol, listing to its side and slowly sinking even as passengers were jumping out or being winched up by helicopters.
At least 87 vessels and 18 aircraft swarmed around the stricken ship. Rescuers clambered over its sides, pulling out passengers wearing orange life jackets.
But the ship overturned completely and continued to sink slowly. Within a few hours only its blue and white bow was seen sticking out of the water. Very soon that too had disappeared.
Some 160 coast guard and navy divers were searching for survivors inside the ship's wreckage, a few kilometres from the shore of Byeongpung Island, which is not too far from the mainland. The area is about 470 kilometres (290 miles) from Seoul.
One student, Lim Hyung-min, told broadcaster YTN after being rescued that he and other students jumped into the ocean wearing life jackets and then swam to a nearby rescue boat.
"As the ferry was shaking and tilting, we all tripped and bumped into each other," Mr Lim said, adding that some people were bleeding. Once he jumped, the ocean "was so cold. ... I was hurrying, thinking that I wanted to live".
Those rescued - wet and many without shoes - were brought to the nearby Jindo Island, where medical teams wrapped them in pink blankets and checked them for injuries before settling them down on the floor of a cavernous gymnasium hall.
The ship had set sail from Incheon, a city in the northern part of the country and the site of South Korea's main international airport, last night for an overnight, 14-hour journey to the tourist island of Jeju.
About three hours from its destination, the ferry sent a distress call at about 9am local time after it began listing to one side, according to the ministry of security and public administration. Officials did not know what caused it to sink, and said the focus was still on rescuing survivors.
Lee Gyeong-og, a vice minister for South Korea's public administration and security ministry, said 30 crew members, 325 high school students, 15 school teachers and 89 non-student passengers were aboard the ship.
Kang Byung-kyu, a government minister, said two of the dead were a female crew member and a male high school student. He said a third body was also believed to be that of a student. A coast guard officer confirmed a fourth fatality but had no immediate details about it.
Mr Kang said 164 people were rescued, of whom 55 were injured. He said 292 people were missing, likely to be either trapped inside the ship or floating in the ocean.
Yonhap news agency said the 146-metre (480ft) long ship, which travels twice a week between Incheon and Jeju, was built in Japan in 1994 and could carry a maximum of 921 people, 180 vehicles and 152 shipping containers.
The water temperature in the area was about 12C (54F), cold enough to cause signs of hypothermia after about one-and-a-half to two hours of exposure, according to an emergency official. Officials said mud on the ocean floor made underwater search operations difficult.
It was not immediately known how deep the ocean floor was.
Passenger Kim Seong-mok told YTN that he was "certain" that many people were trapped inside the ferry as water quickly rushed in and the severe tilt of the vessel kept them from reaching the exits. Some people urged those who could not get out of the ferry to break windows.
Mr Kim said that after having breakfast he felt the ferry tilt and then heard it crash into something. He said the ferry operator made an announcement asking that passengers wait and not move from their places. Mr Kim said he did not hear any announcement telling passengers to escape.
The students - half of them boys and half girls - are from Ansan High School in Ansan city, which is near Seoul, and were on their way to Jeju island for a four-day trip, according to a relief team set up by Gyeonggi Province, which governs the city.
There are faster ways to get to Jeju, but some people take the ferry from Incheon because it is cheaper than flying. Many South Korean high schools organise trips for students in their first or second years. The students on the ferry were in their second year, which would make most of them 16 or 17.
At the high school, students were sent home and parents gathered for news about the ferry.
Park Ji-hee, a first-year student, said she saw about a dozen parents crying at the school entrance and many cars and taxis gathered at the gate as she left.
She said some students in her classroom began to cry as they saw the news on their handsets. Teachers tried to soothe them, saying that the students on the ferry would be fine.
The maritime ministry said the two previous deadliest ferry disasters were in 1970 when 323 people drowned and in 1993 when 292 people died.