A Cambodian court on Friday sentenced top opposition leader Kem Sokha to 27 years in jail for treason, in a case rights groups say is politically motivated.

Kem Sokha was the joint founder of the now-dissolved Cambodia National Rescue Party and has long been a foe of Hun Sen -- Asia's longest-serving leader.

"Kem Sokha... is sentenced to 27 years in prison on the charge of collusion with foreigners committed in Cambodia and other places," Judge Koy Sao said at the court in Phnom Penh.

Immediately after the verdict, the 69-year-old was placed under house arrest, where he will be banned from meeting anyone who is not a family member.

He has one month to appeal the conviction and jail sentence, Ang Udom, a lawyer for Kem Sokha told reporters.

The court also stripped him of his right to vote and barred him from running for political office.

Arrested in 2017 in a midnight swoop involving hundreds of security forces, Kem Sokha was accused of hatching a "secret plan" in collusion with foreign entities to topple the government of longtime ruler Hun Sen.

He has repeatedly denied the charges against him.

Critics say Hun Sen has wound back democratic freedoms and used the courts to stifle opponents, jailing scores of opposition activists and human rights defenders.

"I cannot accept this ruling," Kem Sokha supporter Chea Samuon told AFP outside the courtroom.

"It is very unjust for him and the people. He is not guilty, this is political pressure."

US Ambassador to Cambodia W. Patrick Murphy, who was at the court, slammed the trial and sentence as a "miscarriage of justice."

Last August, US Secretary of State Antony Blinken met Kem Sokha during a visit to Phnom Penh, where he also raised concerns about the kingdom's ailing democracy in talks with Hun Sen.

- One-party state -
Rights groups say Hun Sen has wound back democratic freedoms and created a climate of fear in the country.

Two months after Kem Sokha's arrest, Cambodia's Supreme Court dissolved the CNRP, once considered the sole viable opponent to the ruling Cambodian People's Party (CPP).

That paved the way for the CPP and Hun Sen to win all 125 parliamentary seats in 2018, turning the country into a one-party state.

Scores of opposition figures were convicted of treason last year, some in absentia -- the latest squeeze on opponents ahead of elections.

Last month, Hun Sen ordered the shutdown of one of the country's few remaining local independent media outlets after taking issue with a news report about his son.

Kem Sohka's trial illustrated the "frightening problem of the state control of the judiciary in the country", said Chak Sopheap, executive director of the Cambodian Center for Human Rights.