Huge crowds had gathered on a small island for the final day of the Water Festival, one of the main events of the year in Cambodia.

Panic broke out after a concert on Diamond Island, which followed a boat race on the Tonle Sap river regarded as a highlight of the festivities.

Calmette Hospital, Phnom Penh's main medical facility, was filled with dead bodies as well as the injured, some of whom had to be treated in hallways.

Prime Minister Hun Sen described the stampede as the "biggest tragedy" to hit Cambodia since the mass killings carried out by the Khmer Rouge regime in the 1970s.

The PM ordered an investigation and declared a national day of mourning for later in the week. He also asked all government ministries to fly the nation's flag at half-mast.

Government spokesman Khieu Kanharith said that more than 400 people were injured.

"Most of the deaths were as a result of suffocation and internal injuries," he said.

Authorities estimated that more than two million people would attend the three-day festival.