Opposition leader Sam Rainsy, who is now in France, was convicted and sentenced in absentia on Wednesday to two years in prison for uprooting border markings with neighbouring Vietnam and for inciting racial discrimination.

Two villagers were also found guilty of intentionally damaging temporary border posts during the incident in October and each were jailed for a year.

New York-based Human Rights Watch said the court's closed-door trial was "a farce" that took Hun Sen's campaign of "persecution of critics to a new extreme and highlights government control over the judiciary".

"The Cambodian government's relentless crackdown on critics continues apace in 2010," said Brad Adams, HRW Asia director, in a statement.

"Hun Sen seems intent on reversing the political pluralism that has been created over the past two decades," he added.

The Cambodian Centre for Human Rights, a local rights group, said this week that Sam Rainsy's conviction "reflects Cambodia's rotten democracy".

Cambodia and Vietnam officially began demarcating their contentious border in September 2006 in a bid to end decades of territorial disputes.

That has led to a row that has sparked virulent anti-Vietnamese sentiment in Cambodia, fuelled by resentment of Vietnam's expansion over the centuries and the feeling that Cambodia is losing some of its territory.

The 1,270-kilometre (787-mile) border has remained essentially unmarked and vague since French colonial times.

French-educated former finance minister Sam Rainsy is the main rival to Hun Sen. He has promised to promote liberal democracy and human rights, raise wages and fight corruption if elected, but is often criticised for inciting populist anti-Vietnamese sentiment.