The death toll from a stampede on a bridge in Cambodia's capital rose to at least 339 people with nearly just as many wounded, the prime minister said.

Most of the victims were crushed or drowned, after thousands panicked when several people were electrocuted while celebrating the end of an annual water festival.

Prime Minister Hun Sen apologised on television for the disaster.

State television said two hospitals reported 278 were killed, among them 240 women.

Many people died after leaping from the bridge across a tributary of Tonle Sap river in Phnom Penh, authorities said.

Mr Hun said the death toll is the country's ‘biggest tragedy’ since the Khmer Rouge.

‘This is the biggest tragedy since the Pol Pot regime,’ Hun Sen said in a live television broadcast early Tuesday, referring to the leader of the brutal regime that ruled between 1975 and 1979 and left up to a quarter of the population dead.

At least 339 people had died in Monday's stampede, and more than 300 were injured, the premier added. It remains unclear what triggered the incident.

Mr Hun also said Cambodia would hold a national day of mourning on Wednesday.

Millions of people were out on the streets of the capital to enjoy the final day of the three-day Water Festival, which marks the reversal of the flow between the Tonle Sap and Mekong rivers.