A panic-stricken crowd - celebrating the end of the rainy season on an island in a river - tried to flee over a bridge and many people were crushed underfoot or fell over its sides into the water. Disoriented victims struggled to find an escape hatch through the human mass, pushing their way in every direction. After the stampede, bodies were stacked upon bodies on the bridge as rescuers swarmed the area.
The prime minister's special adviser, Om Yentieng, denied some reports that the victims were electrocuted by lighting cables and that the panic was sparked by a mass food poisoning.
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. Relatives, some crying, searched for the missing Tuesday morning.
"I was taken by shock. I thought I would die on the spot. Those who were strong enough escaped, but women and children died ," said Chea Srey Lak, a 27-year-old woman who was knocked over by the panicked crowd on the bridge.
She managed to escape but described a woman, about 60 years old, lying next to her who was trampled to death by hundreds of fleeing feet.
"There were cries and calls for help from everywhere, but nobody could help each other. Everyone just ran," she said at Calmette Hospital, where she was being treated for leg and hand injuries.
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. Hundreds of shoes were left behind on and around the bridge. An Associated Press reporter saw one body floating in the river.
The government television station said 349 people had been killed and 500 injured.
"This is the biggest tragedy we have experienced in the last 31 years, since the collapse of the Khmer Rouge regime," Prime Minister Hun Sen said, referring to the ultra-communist movement whose radical 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, the Bon Om Touk, which marks the end of the rainy season and whose main attraction is traditional boat races along the river. In this year's event, 420 of the long, sleek boats competed, with crews of up to 80 racers each.
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 and exhibition were 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." Heijmans works for the Cambodia Daily, a local English-language newspaper.
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. Hun Sen called on foreign investors and tourists not to shun the country because of the accident.
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.