PHNOM PENH, Cambodia >> Thousands of Cambodians celebrating a water festival on an island in a river in the capital stampeded Monday night, leaving more than 300 people dead and hundreds injured.

Some in the panicky crowd who tried to flee over a bridge were crushed underfoot or fell over its sides into the water.

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. An Associated Press reporter saw one body floating in the river, and hundreds of shoes left behind on and around the bridge.

Prime Minister Hun Sen, in the third of three post-midnight live television broadcasts, said that 339 people had been killed and 329 injured. He described the chaos as the “biggest tragedy” to strike his country since the communist Khmer Rouge ruled in a reign of terror in the 1970s, and ordered an investigation.

Hun Sen declared Thursday would be a national day of mourning, and ordered all government ministries to fly the flag at half-staff.

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.

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.