A Phnom Penh court ruled that Mu Sochua of the Sam Rainsy Party was guilty of defaming Hun Sen for remarks she made during an April press conference, in which she also announced she would sue the premier for allegedly insulting her.
After Tuesday's ruling, opposition leader Sam Rainsy and Mu Sochua, along with dozens of her supporters, marched from the court through the capital, chanting, "Justice! Justice!"
Police attempting to break up the march clashed with a few protesters, grabbing, punching and kicking several of the loudest demonstrators, an AFP correspondent at the scene witnessed.
Most journalists and rights activists were banned from attending the brief hearing in which Judge Sem Sakola ordered Mu Sochua to pay 2,000 dollars in compensation to Hun Sen, who filed the suit soon after her April remarks.
The judge also ordered the lawmaker to pay a fine of 2,125 dollars to the state, but Mu Sochua told reporters that she did not accept the court's decision.
"The ruling is based on political motivation, not by laws," Mu Sochua said outside court.
"That was not justice. I made it very clear, I will continue to fight until justice is provided to me."
The Cambodian National Assembly stripped Mu Sochua of parliamentary immunity in June to clear the way for the controversial court proceedings against her.
Her lawsuit against Hun Sen was meanwhile dismissed in court.
International rights groups have expressed alarm over a recent spate of defamation and disinformation lawsuits lodged by senior Cambodian officials against government critics.
New York-based Human Rights Watch alleged Hun Sen's government aimed to silence political opposition and critics with a recent "campaign of harassment, threats, and unwarranted legal action."
Sam Rainsy was stripped of his parliamentary immunity in February after accusing Hun Sen's party of corruption during elections last year and then failing to pay a 2,500-dollar fine.
His immunity was later restored when his party eventually paid the fine.