FILE PHOTO: Cambodia's opposition leader and President of the CNRP Kem Sokha talks during an interview with Reuters in Prey Veng

Cambodian Prime Minister Hun Sen threatened on Monday that the 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’s arrest on Sept. 3 marked an escalation in a crackdown on Hun Sen’s critics ahead of an election next year in which he could face the toughest electoral challenge in more than three decades in power.

Western countries have criticised the arrest and Kem Sokha’s Cambodia National Rescue Party (CNRP) has said the charges were nonsense and that it would continue to support him and not change its leader.

“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 told a graduation ceremony in Phnom Penh, saying it could mean: “the dissolution of the party”.

Parliament voted on Monday to allow the prosecution of Kem Sokha, who was arrested on Sept. 3.

His party boycotted the parliamentary vote, but it passed easily because Hun Sen’s Cambodian People’s Party (CPP) has a majority. The vote was passed with 67 out of 123 voting in favour. None opposed it.

The vote was specifically on whether to prosecute Kem Sokha and it was unclear what it meant for the status of the parliamentary immunity from prosecution that he technically gets as an elected member of parliament.

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.

Parliamentarians from the opposition party 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 his 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. “We ask for the release of the party president Kem Sokha immediately and without conditions.”

Security was increased at the prison, several hours drive from the capital, Phnom Penh, but no attempt was made to stop the parliamentarians from speaking.

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.

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 after that and a 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.