A stampede during a Cambodian festival with millions of people in attendance left hundreds dead, the country's government said Monday. The death toll is expected to rise.

Officials said more than 4 million onlookers gathered for the Water Festival near Cambodia's royal palace in Phnom Penh.

The incident began around 10 p.m. local time when police fired a water cannon onto a bridge filled with people in an effort to get them to move.

"That just caused complete and utter panic," said Steve Finch, a journalist with the Phnom Penh Post.

At least 339 people were killed in the resulting stampede.

Many fell over the sides of the bridge while trying to flee. Others were trampled to death.