By Prak Chan Thul
PHNOM PENH (Reuters) - Cambodia's main opposition party will boycott a parliamentary vote to strip their detained leader of immunity on Monday and will instead go to Kem Sokha's jail to demand his release, one of his deputies said on Sunday.
Kem Sokha, the head of the Cambodian National Rescue Party (CNRP), was arrested a week ago and charged with treason for allegedly plotting to win power with the support of the United States, escalating a crackdown on critics of Prime Minister Hun Sen and independent media ahead of a general election next year.
The arrest has been criticized by Western countries, but China has backed Hun Sen's government.
Parliament is due to vote on whether to remove the immunity from prosecution which Kem Sokha gets as an elected member of parliament. The ruling Cambodian People's Party (CPP)'s majority means the motion is certain to pass anyway.
CNRP deputy leader Mu Sochua said the parliament vote was illegal.
"We can't accept this. We will demand that Kem Sokha, who has not done anything wrong, to be released," she said, adding that members of parliament would hold a protest at the prison where he is being held near the border with Vietnam.
CPP spokesman Sok Eysan said the opposition boycott would be unconstitutional, but the CPP had enough votes to strip Kem Sokha of his immunity anyway.
Hun Sen, a 65-year-old former Khmer Rouge commander, has ruled Cambodia for more than 30 years and said last week that he planned to stay in power for another decade.
Next year's election had been expected to be his toughest electoral test, but Western countries and human rights groups have raised doubts as to whether the vote will be fair.
The evidence presented against Kem Sokha so far is a video recorded in 2013 in which he discusses a strategy to win power with the help of unspecified Americans. His lawyers have dismissed the evidence as nonsense and said he was only discussing election strategy.
In recent weeks, Hun Sen has also expelled the National Democratic Institute, a non-governmental organization that promotes democracy, and ordered 19 radio stations off the air.
The independent English-language Cambodia Daily shut last week after being given a month to pay a $6.3 million tax demand which it believed was politically motivated.
The paper's 86-year-old American founder, Bernard Krisher, said in a letter to Hun Sen and made available to media that he planned to travel to Cambodia from his home in Japan to take responsibility for the tax dispute.
(Editing by Matthew Tostevin and Christian Schmollinger)