PHNOM PENH, Cambodia - Thousands of people stampeded during a festival in the Cambodian capital, leaving at least 345 dead and hundreds injured in what the prime minister called the country's biggest tragedy since the 1970s.

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Some in the panicky crowd - who were celebrating the end of the rainy season on a sliver of land in a river - tried to flee over a bridge and were crushed underfoot or fell over its sides into the water. A witness who arrived shortly after the stampede Monday night described "bodies stacked on bodies" on the bridge as rescuers swarmed the area.

Ambulances raced back and forth between the river and the hospitals for several hours after the stampede. Calmette Hospital, the capital's main medical facility, was filled to capacity with bodies as well as patients, some of whom had to be treated in hallways. Many of the injured appeared to be badly hurt, raising the prospect that the death toll could rise as local hospitals became overwhelmed.

Hours after the chaos, the dead and injured were still being taken away from the scene, while searchers looked for bodies of anyone who might have drowned. A reporter saw one body floating in the river, and hundreds of shoes left behind on and around the bridge.

Prime Minister Hun Sen, speaking Tuesday morning, said that 345 people had been killed and more than 320 injured. He described the chaos as the biggest tragedy to strike his country since the rule of the Khmer Rouge, whose policies are blamed for the deaths of 1.7 million people during the 1970s.

He ordered an investigation into the cause of the stampede and declared Thursday would be a national day of mourning. Government ministries were ordered to fly the flag at half-staff. He said that the government would pay the families of each dead victim 5 million riel ($1,250) for funeral expenses and provide 1 million riel ($250) for each injured person.

Authorities had estimated that upward of 2 million people would descend on Phnom Penh for the three-day water festival, which marks the end of the rainy season and whose main attraction is traditional boat races along the river.

The last race ended early Monday evening, the last night of the holiday, and the panic started later on Koh Pich - Diamond Island - a long spit of land wedged in a fork in the river where a concert was being held. It was unclear how many people were on the island to celebrate the holiday, though the area appeared to be packed with people, as were the banks.

Soft drink vendor So Cheata said the trouble began when about 10 people fell unconscious in the press of the crowd. She said that set off a panic, which then turned into a stampede, with many people caught underfoot.

Information Minister Khieu Kanharith gave a similar account of the cause.

Seeking to escape the island, part of the crowd pushed onto a bridge, which also jammed up, with people falling under others and into the water. So Cheata said hundreds of hurt people lay on the ground afterward. Many appeared to be unconscious.

Philip Heijmans, a 27-year-old photographer from Brooklyn, New York, who arrived at the scene half-an-hour after the stampede, walked up the bridge to see hundreds of shoes and pieces of clothing, then a body, then more "bodies stacked on bodies."

He counted about 40 in all, with about 200 rescuers in the area. Some Australian firefighters were on the scene - it wasn't clear why they were in town - who were checking pulses before loading bodies into vans.

Cambodia is one of the region's poorer countries, and has an underdeveloped health system, with hospitals barely able to cope with daily medical demands.

Koh Pich used to host a slum community, but in recent years the poor have been evicted to make way for high-rise and commercial development, most yet to be realized.