A stampede during Cambodia's annual water festival in Phnom Penh left at least 339 people dead Monday, according to Cambodian officials.

When the stampede occurred, the crowd had reached a staggering 4 million, said Visalsok Nou, a Cambodian Embassy official in Washington. Organizers expect about 2 million people every year.

 Mass panic ensued when 10 festival-goers standing in close proximity to one another collapsed or became unconscious, which caused the crowd to stampede. The entire crowd headed toward a bridge that connected the island to the mainland, but the bridge collapsed, throwing people into the water. 

 Video footage showed chaos in the streets and a flooding of people into local hospitals.

In addition to the 339 dead, hundreds more were injured. The streets were littered with personal belongings, clothing and other items.

"With this miserable event, I would like to share my condolences with my compatriots and the family members of the victims," said Prime Minister Hun Sen in response to the tragedy.

He has ordered an investigation of the incident and said the country will hold a national day of mourning later this week.

The water festival is a three-day annual event to celebrate the end of Cambodia’s rainy season and a 12th-century victory of the Cambodian naval forces.

Reach staff reporter Melissa Dempsey here.

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