Cambodian opposition figure Kem Sokha was sentenced to 27 years in prison Friday for treason in a case criticized by the United States as a miscarriage of justice.

The Phnom Penh Municipal Court banned the 69-year-old head of the now-dissolved Cambodia National Rescue Party from conducting political activities, including voting and traveling overseas, allowing him to stay under house arrest until legal procedures are over, including a possible appeal through the Supreme Court.

Kem Sokha was arrested in September 2017 at his home in Phnom Penh on suspicion of attempting to topple the government of Prime Minister Hun Sen, who has been in power since 1985. Kem Sokha has denied the allegations, saying he has not conspired with foreign powers, and called for the charges to be dropped.

Cambodian opposition figure Kem Sokha waves to the press in December 2022 in Phnom Penh. (Kyodo)

Following his arrest, Kem Sokha's opposition party was forcibly dissolved in November 2017. In a lower house election in July 2018, the ruling Cambodian People's Party won all 125 seats, allowing Hun Sen to extend his rule for another five years.

His trial began on Jan. 15, 2020, and it took more than three years for the court to complete dozens of hearings before presiding judge Kouy Sao handed down the ruling.

U.N. human rights experts have expressed concerns about the trial, saying the entire process of his arrest and detention has been "tainted" by irregularities.

Following the ruling, U.S. Ambassador to Cambodia Patrick Murphy issued a statement saying the United States is "deeply troubled" by the conviction of Kem Sokha.

"His trial, built on a fabricated conspiracy, was a miscarriage of justice," it said.

During the trial, prosecutors submitted, as proof of Kem Sokha's alleged collusion with foreign powers, a short video clip of him telling party supporters in Australia in 2013 he had received advice from Washington on building an opposition movement.

Kem Sokha and his defense team repeatedly argued that the full video footage should be relied upon instead of edited clips.

Both prosecutors and lawyers representing the government have refrained from naming the foreign states allegedly involved.

The United States said it has provided $3 billion in assistance to Cambodia in recent decades, including transparent assistance to strengthen institutions and political parties in line with Cambodia's Constitution.

"Our programs have been available for all political parties, including the governing CPP party, and many government institutions," Murphy has said.

"We do, however, support freedom of expression, an active civil society," he added.

Chan Thoeun, a supporter of Kem Sokha who waited outside the court, said the ruling was regrettable, and suggested that the charges against Kem Sokha be dropped and his release granted.

"By doing so, a national reconciliation can be achieved, which is in the interests of the nation and its people," he said.

Following his arrest, Kem Sokha's opposition party was forcibly dissolved in November 2017. In a lower house election in July 2018, the ruling Cambodian People's Party won all 125 seats, allowing Hun Sen to extend his rule for another five years.