PHNOM PENH (REUTERS) - Cambodia's main opposition party said on Tuesday (Sept 12) that it would stand in next year's election despite the treason charges against its leader and a threat by Prime Minister Hun Sen that it could be dissolved.

Hun Sen, a former Khmer Rouge commander who has ruled for more than 30 years, said on Monday that the Cambodia National Rescue Party (CNRP) would be dissolved if it continues to back detained leader Kem Sokha.

Kem Sokha was arrested on Sept 3, and has been charged with treason over an alleged plot to gain power with US support. He is the only serious election rival to Hun Sen, who is one of China's closest allies in South-east Asia.

In a news briefing on Tuesday, senior CNRP member Son Chhay said the opposition would not boycott the July 2018 general election in which it faces Hun Sen's ruling Cambodian People's Party (CPP).

"The CNRP will go into the 2018 election despite the enormous difficulties," Son Chhay said. "The only choice is to go into the upcoming election."

Western countries have criticised the arrest of Kem Sokha, which marked an escalation in a crackdown on critics ahead of a poll next year that could pose the toughest electoral challenge the 65-year-old Hun Sen has faced during his long rule.

On Monday, one of the CNRP's deputy presidents, Mu Sochua, had raised the possibility of an election boycott if Kem Sokha is not freed, but she said on Tuesday that she agreed with Son Chhay that the party should take part.

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.

On Tuesday, Son Chhay said Kem Sokha was innocent until a final conviction by court, saying that the arrest and a parliament vote to allow his prosecution were illegal.

The ruling CPP party's spokesman, Sok Eysan, said "Kem Sokha committed serious crimes that would lead to the destruction of peace and political stability".