PHNOM PENH (Reuters) - A Cambodian court on Thursday rejected a plea for bail by detained opposition leader Kem Sokha, following his arrest last year on charges of seeking to overthrow the government.

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Kem Sokha, head of the now dissolved Cambodia National Rescue Party (CNRP), was arrested on Sept. 3 amid a crackdown on critics of authoritarian Prime Minister Hun Sen.

Rights groups have criticized the crackdown as Cambodia prepares to hold two elections this year. The CNRP’s dissolution and the jailing of Kem Sokha have removed the biggest challenge Hun Sen may have faced at the polls, the groups say.

A Senate election is set for Feb 25, and a general election is due in July.

“The court decided to uphold the lower court’s decision not to release Kem Sokha on bail,” Touch Tharith, a spokesman for the Appeal Court, told Reuters.

He declined to say why the application was rejected.

Lawyers for Kem Sokha, who sought bail on the grounds that he suffers from high blood pressure and diabetes, said judges cited security concerns as a reason for the rejection.

“I am disappointed that the court didn’t consider these illnesses ... and continued to detain him,” Chuong Chu Ngy, a lawyer for Kem Sokha, told reporters.

Lawyers for Kem Sokha said he appeared in court on Thursday.

No date has been set for the trial.

Dozens of Kem Sokha’s supporters gathered outside the court and his 92-year-old mother was seen weeping.

Observers from the Australian, Swedish and United States embassies and the U.N. human rights office were denied access to the hearing.

The CNRP was dissolved by Cambodia’s Supreme Court in November at the request of the government.

Hun Sen has accused Kem Sokha of getting help from the United States to overthrow the government. The U.S. embassy has rejected any suggestion of interference in politics.

In a speech on Thursday Hun Sen said nobody could topple him.

“Nobody can topple Hun Sen, except Hun Sen,” he said, adding that any group that tried to remove him from power would be met with deadly force.