Cambodia's most prominent opposition leader has been sentenced to 27 years under house arrest for treason.

Kem Sokha, the former leader of the now-dissolved Cambodian National Rescue Party, has also been barred from standing or voting in elections.

He was accused of conspiring with foreign powers to overthrow Prime Minister Hun Sen's government.

Hun Sen is one of the world's longest-serving autocrats, who has been in power since 1985.

Cambodia will hold a general election in July. Most expect he will run again, although some believe he is planning to hand over power to his eldest son, Hun Manet.

Kem Sokha, 69, was first arrested in 2017, based on a 2013 video where he said he had got support from US pro-democracy groups.

He and his lawyers have denied the charges and said the case against him was baseless.

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

Kem Sokha's lawyer, Ang Udom, said his legal team would appeal against the verdict, Reuters news agency reported.

Human rights groups and Western governments have condemned the charges against him as unfounded and politically motivated.

"It was obvious from the start that the charges against Kem Sokha were nothing but a politically motivated ploy by Prime Minister Hun Sen to sideline Cambodia's major opposition leader and eliminate the country's democratic system," said Phil Robertson, deputy Asia director at Human Rights Watch.

"Sending Kem Sokha to prison isn't just about destroying his political party, but about squashing any hope that there can be a genuine general election in July."

The group called on the authorities to quash the conviction and "immediately and unconditionally" release him.

Few will be surprised by this verdict, as the Cambodian courts are widely seen as being firmly under the control of Hun Sen.

Kem Sokha, a former human rights advocate, led the Cambodian National Rescue Party. Its popularity surged in past elections, to a point where it was poised to unseat Hun Sen and his party.

His party was banned ahead of a 2018 election. Most of its leaders have either been prosecuted or forced into exile.

Last month, Hun Sen shut down one of the country's last independent media outlets, Voice of Democracy, saying that it published a story that attacked his son and damaged the government's reputation.