PHNOM PENH (REUTERS) - A Cambodian court rejected an appeal to release opposition leader Kem Sokha on Tuesday (Sept 26) and said he would not be granted bail as he awaits trial on treason charges.

The arrest of Kem Sokha on Sept 3 came amid a crackdown on critics of authoritarian Prime Minister Hun Sen, who could face the biggest electoral challenge of more than three decades in power next year.

Lawyers for Kem Sokha, leader of the Cambodia National Rescue Party (CNRP), had asked the Appeal Court to rule that his detention was illegal on the grounds that he should get immunity from prosecution as an elected member of parliament.

"The Appeal Court decided to uphold the lower court decision to detain Kem Sokha," court spokesman Touch Tharith told Reuters, adding that he would also not be granted bail.

No date has been set for the trial.

Hundreds of Kem Sokha's supporters gathered at barricades outside the Appeal Court ahead of the hearing, but police did not bring him to court - prompting his lawyers to walk out before the court delivered its verdict.

"According to national and international laws, the accused must be present to defend his rights," said lawyer Chan Chen."Without the presence of our client, we can't participate in this hearing."

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 US experts.

Mr Hun Sen says his rival was getting help from the United States. The US embassy has rejected any suggestion of interference in politics.

The CNRP on Monday put up banners around Cambodia calling for the release of Kem Sokha in a challenge to the prime minister, who has said the party may be dissolved if it does not drop him and choose another leader.

Mr Hun Sen has said he plans to stay in power for another decade.