The tragedy occurred during the high point of the Water Festival
At least 339 people been killed in a stampede during festival celebrations in the Cambodian capital, Phnom Penh, Prime Minister Hun Sen has said.
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.
The stampede took place on a bridge, which eyewitnesses said had become overcrowded.
Hundreds more people were injured in the crush.
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.
He said he had ordered an investigation and declared a national day of mourning for later in the week. He ordered all government ministries to fly the nation’s flag at half-mast.
Authorities had estimated that more than two million people would attend the three-day festival.
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.
Sean Ngu, an Australian who was visiting family and friends in Cambodia, told the BBC too many people had been on the bridge.
People at both ends were pushing, he said, causing those in the middle to fall to the ground and then get crushed.
The bridge became jammed with people, some crushed under foot and others falling into the water.
Witnesses spoke of bodies littering the area.
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.