Phnom Penh, Cambodia – Cambodia’s Supreme Court upheld a ruling to keep opposition leader Kem Sokha imprisoned in the run-up to his trial on treason charges.

Kem Sokha – the president of the Cambodia National Rescue Party (CNRP) – was arrested on September 3 and accused of conspiring with the United States to launch a coup against the government of Prime Minister Hun Sen.

The CNRP has denied the allegations, calling the arrest politically motivated with an election upcoming in 2018.

“Putting Mr Kem Sokha on trial is a mockery of justice as the judiciary as well as police and military forces are in the hands of Prime Minister Hun Sen,” opposition official Mu Sochua – who fled Cambodia this month after receiving arrest threats – told Al Jazeera.

“What is at stake is the life of democracy in Cambodia, which is hanging on a tightrope as Mr Hun Sen sees no way out of his 32 years in power but to destroy all democratic forces,” said Mu Sochua.

Kem Sokha wasn’t allowed to attend Tuesday’s appeal hearing at the Supreme Court in the capital because of “security concerns”. Police in Phnom Penh was on high alert with authorities saying they would not allow any “anarchic protests”.

Since Kem Sokha’s arrest, at least half of the opposition members in parliament have fled Cambodia.

Dozens of independent media were forced to close down by the government, local NGOs have been censured, and the US-funded National Democratic Institute was ordered to cease operations.

Hun Sen has called those caught up in the crackdown, “rebels” who are trying to overthrow his government.

The charges against Kem Sokha stem from a video recorded in 2013 of a speech he made in Australia to supporters. In it, he discussed how the US had advised him on political strategy.

Rights groups and many in the international community say the video is not enough to demonstrate his guilt of the crime of treason. If convicted, he faces 30 years in prison.

The Supreme Court will also rule whether to dissolve the CNRP on November 16.

Charles Santiago, chair of the ASEAN Parliamentarians for Human Rights, denounced the government’s recent moves as heavy-handed.

“This crackdown has reached unprecedented heights and shows no signs of reversing course… Cambodia’s democratic aspirations are being smothered by a government that knows it has lost the support of the people yet remains bent on clinging to power,” Santiago told Al Jazeera.