The protesters, demonstrating since last week for higher wages, clashed with riot and military police outside of an industrial park, site of many garment factories, in southwestern Phnom Penh,CNN reported.
Protesters threw rocks, used slingshots and built barricades by collecting scrap and lighting it on fire, witnesses said. They said security forces responded by firing live ammunition.
Chuon Narin, Phnom Penh's deputy police chief, said three people were killed and two wounded but a local human rights group, Licadho, said it thought as many as 23 people were wounded.
Chan Dy, a garment worker in the industrial park, told CNN he was "very worried because many workers were injured."
At the Khmer-Soviet Friendship Hospital, where some of the wounded were taken, Dr. Chin Kosal said intensive care staff members were treating six gunshot victims and one other wounded person.
There were reports of many arrests.
Garment workers are striking to demand an increase in their minimum wage from $80 to $160 a month. They have rejected government offers of increases to $95 and $100. Cambodia's garment industry is estimated to employ 400,000 people and account for roughly $5 billion in exports.