PHNOM PENH (Reuters) - Cambodian Prime Minister Hun Sen threatened on Monday that the main opposition party would be dissolved if it continues to back detained leader Kem Sokha, who has been charged with treason over an alleged plot to gain power with U.S. support. Kem Sokha was arrested on Sept. 3 and is the only serious election rival to Hun Sen, a 65-year-old former Khmer Rouge commander. Western countries have criticized the arrest, which marked a an escalation in a crackdown on critics ahead of a poll next year that could pose the toughest electoral challenge Hun Sen has faced in more than 30 years of rule.  The opposition Cambodia National Rescue Party (CNRP) said it would continue to support Kem Sokha as leader and threatened to boycott the election if he is not freed. Speaking at a graduation ceremony in Phnom Penh, Hun Sen warned that the CNRP s stand could mean  the dissolution of the party .  If the political party continues to blockade and defend this traitor, it means the party is also a traitor so there is no time to let this party operate in Cambodia s democratic process anymore,  Hun Sen said. Parliamentarians from the CNRP went to the prison where Kem Sokha is being held to demand his release. They said his arrest was illegal because he should have been protected by parliamentary immunity.  The party president Kem Sokha is the CNRP president now and will be in the future,  one of his deputies, Mu Sochua, said outside the prison, adding that his release was an essential condition to allow a free and fair election.  We can t participate in an election that isn t free and fair,  she said. The opposition party boycotted a parliamentary vote on whether Kem Sokha should be prosecuted. It would not have been able to block approval as Hun Sen s Cambodian People s Party (CPP) holds a majority, and the motion in favor of prosecuting was passed unopposed. It was unclear whether that effectively overrode Kem Sokha s right to claim parliamentary immunity. 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. Western countries and human rights groups have condemned the arrest of Kem Sokha and raised doubts as to whether next year s election can be fair, given the crackdown on the opposition, activists and independent media.   However Hun Sen s main ally, China, has said it supports Cambodia s efforts to preserve its own security. Hun Sen was due to visit Beijing on Monday. He said he was going to ask for more aid for Cambodia s health sector. (Story refiles to correct opposition party acronym to CNRP in later references.) 