Millions flock to the banks of Tonle Sap and Mekong rivers to watch boat races during the annual celebrations.
Grieving relatives in the Cambodian capital are preparing for funerals of 351 revellers killed in a stampede on an overcrowded bridge.
The deaths occurred on Monday night as tens of thousands of people tried to flee an island in the Bassac River across a narrow bridge during a traditional festival in Phnom Penh.
The cause of the surge is unclear and the authorities are investigating why throngs of revellers panicked at the annual water festival, crushing and trampling people underfoot.
Thousands of Cambodians lit candles on Wednesday and made offerings to appease the souls of those who perished.
One woman said that people believed that after a tragedy on such a scale the souls of the dead would gather in the city and may wreak harm if not properly appeased.
“I asked their souls to rest in peace and not to be angry with those still alive in the capital, especially my family members and relatives,” Meng Houth, who laid out a banana, cans of rice and salt along with incense and a candle in front of her home, said.
‘Sense of shock’
Hun Sen, the Cambodian prime minister, has called the incident one of the country’s worst disasters ever.
“This is the biggest tragedy since the Pol Pot regime,” he said in a live television broadcast, referring to the leader of the brutal regime that ruled the country between 1975 and 1979 and left up to a quarter of the population dead.
The prime minister has declared Thursday a national day of mourning, and ordered all government ministries to fly the flag at half-mast.
Police were looking for clues on the bridge to what triggered people to panic at the annual water festival and caused the stampede on an overcrowded narrow bridge in Phnom Penh.
Al Jazeera’s Harry Fawcett, reporting from Phnom Penh, said the bridge was overcrowded and when it started swaying it sent people panicking.
“It is believed that there were simply so many people on the bridge that it was swaying and that caused people to panic and try to get off all at once,” he said.
“There is a real sense of shock that such a huge number of people died at the end of what is an annual happy festival.
“People are falling back on their traditional, religious observance. Large number of students are expected to march here to pay tributes early morning on Thursday.”
Om Yentieng, the prime minister’s special adviser, denied reports that the stampede was sparked by a mass food poisoning, or by people being electrocuted by electric cables.
The government admitted it had overlooked issues of crowd control at the three-day event, which attracted some three million revellers to the capital from all over Cambodia.
“We were concerned about the possibilities of boats capsizing and pick-pocketing. We did well, but we did not think about this kind of incident,” Khieu Kanharith, government spokesman, told the AFP news agency.
Cambodians have been critical of the emergency services’ slow response to the tragedy.