By Agence France-Presse
Tep Vanny, one of the country's best known activists, was convicted of causing violence during a 2013 protest outside strongman Prime Minister Hun Sen's home.
She and her supporters were demonstrating against the imprisonment of another activist following the forced eviction of some 4,000 families in Phnom Penh for a lucrative real estate development.
They say they were attacked by security forces but Phnom Penh Municipal Court ruled that the demonstrators instigated the violence.
Rights groups have long accused Hun Sen of using pliant courts to attack his critics.
After the verdict was handed down, Vanny and her supporters shouted out "injustice" multiple times.
Several dozens of her supporters also briefly scuffled with security guards outside the court, leaving a few people with minor injuries, according to activist Am Sam Ath, of local rights group Licadho.
He described Vanny's conviction as the latest move to "suppress people who are actively leading land protests" against the government.
In the last year Cambodia's courts have ramped up prosecutions of government critics with Licadho classifying at least 25 people as political prisoners.
Vanny was close to the end of serving a previous six-month jail sentence for cursing public officials in 2011 and has previously done time for her activism.
Land evictions are one of the foremost human rights issues in Cambodia, with residents' claims over land routinely ignored and protests frequently facing violent crackdowns by authorities.
The communist Khmer Rouge abolished land ownership during its 1975-1979 rule and many legal documents were lost during the period, complicating land claims today.
Hun Sen has run Cambodia for more than three decades. He defends his rule as one that has brought stability to the country.
But critics say the country remains endemically corrupt, authoritarian and a place of huge inequality with Hun Sen, his family and friends becoming enormously wealthy over those years.