The dead and injured are being taken from the scene where at least 345 people died in a stampede during a water festival in Phnom Penh, Cambodia.

Thousands of Cambodians celebrating a water festival by the river in the Cambodian capital stampeded Monday night. The panic was exacerbated as the crowd rushed to cross a bridge and some fell into the water.

Hundreds of people were killed and hundreds more injured after a stampede onto a bridge during the last day of the annual water festival in Phnom Penh on Monday. (Heng Sinith/Associated Press)

Ambulances raced back and forth between the river and the hospital for several hours after the stampede, while onlookers and relatives waited outside.

The death toll seemed likely to rise sharply as many of the injured appeared to be badly hurt and local medical facilities have limited capacity.

Prime Minister Hun Sen, speaking Tuesday morning, said 345 people had been killed and more than 320 injured. He described the chaos as the biggest tragedy to strike his country since the communist rule of the Khmer Rouge, 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-mast.

The prime minister said the government would pay the families of each dead victim five million riel (about $1,250) for funeral expenses and provide one million riel (roughly $250) for each injured person.

Authorities had estimated that upward of two 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 the traditional boat racing along the Tonle Sap River. Monday night marked the end of the holiday.

The last race ended in early evening, and the panic started later on Koh Pich — Diamond Island — a long spit of land 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 was much of the waterfront.

Military police view the bridge where a stampede took place. (Chor Sokunthea/Reuters)

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.

Part of the crowd pushed onto a bridge, which also jammed up, with people falling under others and off the bridge. So Cheata said hundreds of hurt people lay on the ground afterward. Some appeared to be unconscious.

Police and other officials were unable to immediately provide more details, saying they were too busy attending to the injured.

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.