The authoritarian prime minister sued Senator Thak Lany in August over a video clip allegedly showing her accusing Hun Sen of orchestrating the killing of Kem Ley, a prominent government critic.
The brazen daylight murder of the pro-democracy activist in the capital Phnom Penh in July sent fear rippling across a country with a history of political assassinations.
Senator Thak Lany, who is believed to have fled the country shortly after she was charged, was convicted today in absentia of defamation and inciting chaos with her remarks.
A former soldier has been charged with Kem Ley's murder but police have kept a tight lid on the details of their investigation.
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Suspicions that the killing was ordered from higher up continue to simmer in a country where critics have long been silenced by the political elite.
Hun Sen, who has loomed over Cambodia for 31 years, has a history of ruthlessly undercutting his rivals and has intensified a crackdown on critics ahead of 2018 polls.