More than 300 people have died and hundreds more injured in a stampede during the celebration of a water festival in the Cambodian capital of Phnom Penh.

The stampede occurred on Koh Pich -- Diamond Island -- a small island in Phnom Penh's Tonle Sap river, where a concert was being held after the end of traditional boat races.

The area was packed with young people. Witnesses told the Associated Press that the trouble began when about 10 people fell unconscious in the press of the crowd. That set off a panic, which then turned into a stampede.

Part of the crowd pushed onto a bridge, which became jammed with people falling under others. Many of those on the lighted bridge either fell or jumped off the bridge into the water.

Om Yentieng, a special adviser to Prime Minister Hun Sen, denied media reports late Monday that some victims had been electrocuted by lighting cables and that the stampede began when revellers were struck by food poisoning.

Sen made several live broadcasts Monday night to update Cambodians on the rescue efforts and rising death toll. By early Tuesday morning local time, at least 349 people were confirmed dead, with another 500 people injured.

Sen said that it was not immediately clear what triggered the stampede. "This needs to be investigated more," he said, adding that a committee would be set up to examine the incident.

Upward of 2 million people descend on Phnom Penh for the three-day water festival, which marks the end of the rainy season. The festival's main attraction is traditional boat races along the river.

Hours after the chaos, rescuers were still scouring the water, looking for bodies of anyone who might have drowned.

Chea Srey Lak, a 27-year-old woman who was knocked down during the stampede, said the incident left her in shock.

"I thought I would die on the spot," she said at Calmette Hospital, the capital's primary medical facility, where the dead and injured were taken. "Those who were strong enough escaped, but women and children died."

Lak, who suffered leg and hand injuries in the stampede, said several victims cried out for help but it was impossible to assist anyone in the mass of fleeing people.

"This is the biggest tragedy we have experienced in the last 31 years, since the collapse of the Khmer Rouge regime," Sen said, referring to the country's former communist regime that is blamed for the deaths of 1.7 million people in the 1970s.

Sen declared Thursday a national day of mourning for the victims. He also announced that the government will pay the families of the deceased victims 5 million riel (about $1,250) for funeral costs, and 1 million riel ($250) for every injured person.

With a report from the Associated Press