Yesterday's tragedy happened as a panic-stricken crowd of thousands celebrating the end of the rainy season on an island in a river tried to flee over a bridge.
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 each other on the bridge as rescuers rushed to the area.
"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 a national day of mourning. Government ministries were ordered to fly the flag at half-mast.
The prime minister said that the government would pay the families of each dead victim £780 for funeral expenses and provide £156 for each injured person.
His 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 today.
"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 of about 60 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.
The government television station said 349 people had been killed and 500 injured.
Authorities had estimated that more than two 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 yesterday evening local time, 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 seller 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.
Philip Heijmans, a 27-year-old photographer from Brooklyn, New York, who works for the Cambodia Daily, a local English-language newspaper, 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.