PHNOM PENH: Prominent Cambodian opposition figure Kem Sokha was on Friday sentenced to 27 years in detention under house arrest after being found guilty of treason, in a case condemned by the United States as politically motivated.

Judge Koy Sao also told the court in the capital Phnom Penh that Kem Sokha would be barred from running for political office or voting in elections.

He was arrested in 2017 over accusations he was conspiring with the United States to overthrow self-styled strongman Hun Sen, who has ruled Cambodia for nearly four decades.

Kem Sokha, who headed the now disbanded Cambodia National Rescue Party (CNRP), had denied the charges and Washington has dismissed the allegations as "fabricated conspiracy theories".

His lawyer, Ang Udom, said his legal team would appeal the verdict.

W. Patrick Murphy, U.S. ambassador to Cambodia, said on Friday the case was a miscarriage of justice.

"We call on authorities to allow all Cambodians to enjoy...universal human rights of peaceful assembly and free expression and to participate in building a truly democratic system," he told reporters outside the court.

CNRP was banned ahead of a 2018 election that was swept by the Cambodian People's Party (CPP) of Prime Minister Hun Sen.

CNRP has since been decimated, with many of its members arrested or fleeing into exile in what activists say is a sweeping crackdown designed to thwart challenges to CPP's power monopoly.

Cambodia is due to hold elections in July, with the opposition launching the Candlelight Party last year, which largely regroups members of the CNRP.

Many media outlets critical of Hun Sen have also been shuttered and civil dissent crushed in recent years.

Last month, Hun Sen ordered the shutdown of The Voice of Democracy, also known as VOD, one of the last independent local news organizations in the country, saying it had attacked him and his son and hurt the country.

Hun Sen is expected to run in elections in July but he has previously offered support for his eldest son as a potential successor.

His son, Hun Manet, a deputy commander of the Royal Cambodian Armed Forces (RCAF) and joint chief of staff, graduated from the United States Military Academy at West Point in 1999.