Prime Minister Hun Sen said 339 people had been killed and 329 injured. He added that it was not immediately clear what triggered the stampede. "This needs to be investigated more," he said, adding that a committee would be set up to examine the incident.
Media agencies reported that 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.
Steve Finch, a Phnom Penh Post reporter, told CNN that the stampede began when police fired a water cannon onto a bridge to an island in the centre of the river.
The bridge was packed with people and police fired the water cannon in an effort to get them to move, he said.
"That just caused complete and utter panic," he told CNN. He said a number of people lost consciousness and fell into the water; some may have been electrocuted.
The festival, which attracts people from all over Cambodia, is held annually to commemorate a victory by the Cambodian naval forces during the 12th century reign of King Jayvarman VII, according to the Tourism Cambodia website.
Image: Military police examine a bridge where a stampede took place in Phnom Penh
Photograph: Chor Sokunthea/Reuters