LEIGH SALES, PRESENTER: Cambodia has declared a national day of mourning after at least 378 people were killed in a stampede in the capital, Phnom Penh.

Hundreds more were injured when people were crushed on a small island on the final day of the Water Festival.

The country's 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.

South-East Asia correspondent Zoe Daniel reports.

Zoe DANIEL, SOUTH EAST ASIA CORRESPONDENT: In a panicked group they crushed each other to death.

The reason for the stampede is unclear but the deadly results won't be reversed with answers. In the heat, as they were pressed together on a bridge designed for far fewer, they removed their shoes and clothes, left as a grim reminder of their owners.

MAN (translated): I was stuck in the crowd for a long time and it was so hot and I became unconscious.

Zoe DANIEL: Phnom Penh's famous annual Water Festival attracts millions, but this year a big crowd in a confined space along the river became a deadly combination.

On the festival's last night, hundreds crossing the river for shopping and concerts suffocated, were crushed underfoot or jumped over the sides and drowned.

Many more will still be pulled from the river.

It's a scene of death that Cambodia knows all too well in a different context and it's shocked this newly optimistic nation.

HUN SEN, CAMBODIAN PRESIDENT (translated): This is the biggest tragedy we've experienced in the last 31 years, since the collapse of the Pol Pot regime.

Zoe DANIEL: At hospitals around the city today there was confusion and ultimately devastation as people searched for the missing and the dead.

Up to 400 died, most young women out having fun. An investigation is underway.

TOUCH NAROTH, PHNOM PENH POLICE CHIEF (translated): For the moment we cannot release the report because we're waiting to speak to the survivors.

Zoe DANIEL: Australian fireman Paul Hurford runs an NGO in Cambodia to help coordinate major incidents. He and his team were impressed with the organisation of the recovery effort but even for these seasoned specialists the scene was shocking.

PAUL HURFORD, FIREMAN: Personally I found it quite challenging. It was a very large incident and as we see now we've got over 345 casualties - sorry, fatalities - from the event and another 300 plus people seriously injured.

So in any scale, whether we're here in Cambodia or in a developed country in a big city, it's still a major incident and still quite challenging for anyone to deal with.

Zoe DANIEL: In hospitals across the city, those whose loved ones were injured maintain a bedside vigil.

Chia may have brain damage and doctors fear he won't recover to care for his wife and two children again.

OUK CHoeURN, DOCTOR: Cambodia has never seen such a big death toll from such an unnecessary incident.

Zoe DANIEL: Fifteen-year-old Molum saw people dying around him. He couldn't help them.

MOLUM (translated): It was so hot and I couldn't even think about what was happening around me.

Zoe DANIEL: There'll be a national day of mourning on Thursday for a country that's once again grieving for its people.

The floats from this year's water festival are now being pulled down but unfortunately the memories of this year's event will take a long time to fade, for all the wrong reasons.

Zoe Daniel, Lateline.