The clash broke out when 100 police refused to allow the monks to approach the embassy in the Cambodian capital.

They knocked one officer unconscious, Phnom Penh police chief Touch Naruth said.

Police used batons to beat back the monks, who responded by hitting out with water bottles, he said.

The Buddhists had marched to the embassy to hand over a letter calling on Vietnam to free Cambodian monk Tim Sakhorn, who was arrested by the communist country nearly five months ago.

'They wanted to enter the Vietnamese embassy, but police asked them to move back. The monks then beat and kicked the police. The officers had to use force to protect themselves,' Touch Naruth said.

'What the monks did was illegal,' he added.

One of the monks, 20-year-old Thach Mony, said that they simply wanted to drop off their petition calling for the release of Tim Sakhorn and for the return of land that Cambodia claims was seized by Vietnam in 1978.

'The monks just wanted to bring a petition to the embassy, but the police misunderstood us. They blocked us, and they used violence against us,' he said.

Vietnam said in early August that it had arrested Tim Sakhorn on charges of undermining national unity by organising anti-Vietnam demonstrations in neighbouring Cambodia.

He had been the abbot of a Cambodian pagoda, but was defrocked in late June and disappeared amid unconfirmed reports he was detained by Cambodian authorities for deportation to Vietnam. -- AFP