Kem Sokha, head of the outlawed Cambodia National Rescue Party (CNRP), is the latest of a long line of government critics to fall foul of a sweeping crackdown by Prime Minister Hun Sen, who has ruled for nearly four decades and has tightened his grip on power.
Monovithya Kem, deputy director of public affairs at the CNRP, tweeted their party boss had also been barred from politics for life and was banned from communicating with anyone except for family members.
US Ambassador to Cambodia, Patrick Murphy, condemned the conviction.
“His trial, built on a fabricated conspiracy, was a miscarriage of justice. Inclusive democracy would further the Cambodian people’s aspirations for a prosperous society that respects all voices and rights,” he said in a tweet.
Kem Sokha, 69, was first arrested on treason charges in 2017 for “involving foreigners in secret plans against the Kingdom of Cambodia.”
He has consistently denied the accusations and rights groups have described his prosecution as politically motivated.
His lawyer, Ang Udom, said his legal team would appeal the verdict, Reuters reported.
Hun Sen has served as the country’s prime minister since 1985, making him one of the world’s longest serving leaders.
Several independent newspapers and websites have been shut down in recent years while dozens of opposition figures have been jailed or forced into exile.
The CNRP was banned ahead of a 2018 election that was swept by Hun Sen’s ruling party.
Few critical voices remain in the Southeast Asian kingdom.
One of the last remaining independent media outlets, the Voice of Democracy, was closed down by authorities last month.
Additional reporting by Reuters