PHNOM PENH, Cambodia -- Cambodian opposition leader Kem Sokha was on Friday sentenced to 27 years in jail for treason, a verdict that ruled him out of elections this year and was immediately denounced by the United States as a "miscarriage of justice".

Kem Sokha was accused of hatching a "secret plan" in collusion with foreign entities to topple the government of longtime Prime Minister Hun Sen, upon his arrest in 2017.

The 69-year-old co-founder of the now-dissolved Cambodia National Rescue Party (CNRP) has long been a prominent adversary of Hun Sen, who critics say has wound back democratic freedoms and used the courts to stifle opposition.

Kem Sokha has repeatedly denied the charges against him, which rights groups say were designed to bar him from politics ahead of July's elections.

Immediately after the verdict at the Phnom Penh court, he was placed under house arrest and banned from meeting foreigners and anyone who is not a family member without permission of the court.

His daughter Kem Monovithya tweeted his house was now under police surveillance.

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

"The United States is deeply troubled by the conviction of respected political leader Kem Sokha," he told reporters.

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.

Phil Robertson of Human Rights Watch said the verdict shows "authoritarians have won" in Cambodia.

"This is Cambodian democracy hitting rock bottom," he told AFP.

As Kem Sokha was led away from the court he smiled and greeted diplomats attending the court, an AFP reporter said.

"I cannot accept this ruling," supporter Chea Samuon told AFP outside the court, where there was a tight security presence.

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

Kem Sokha has one month to appeal the conviction and jail sentence.

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

'Squashing any hope'

Two months after Kem Sokha's arrest in 2017, 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", said Chak Sopheap, executive director of the Cambodian Center for Human Rights.

Amnesty International said use of the courts to hound opponents of Hun Sen "knows no limits".

"The Cambodian justice system has once again shown its jaw-dropping lack of independence," Amnesty International deputy regional director Ming Yu Hah said.

Sebastian Strangio, journalist and author of "Hun Sen's Cambodia," said the sentencing was part of a push to put pressure on opponents and "geared at securing the CPP the expected victory at the upcoming election."

"This is sending a very strong message that anybody who poses a serious challenge to the CPP will be brought to heel," he told AFP.