Protesters say the assailant was in fact Chhouk Bandith, but the courts have dropped charges against him.
Brother of slain activist Chea Vichea said he no longer believes the judiciary can provide justice for the murder of his brother.
Demonstrators against the Boeung Kak lake development gathered in front of the Phnom Penh court on Monday, demanding charges dropped against a woman and her husband who are activists against forced evictions.
Born Samnang and Sok Samoeun were remanded to 20-year prison sentences by the Appeals Court on Thursday, following their release on the order of retrial in 2009.
Thai authorities say they plan to kick out illegal workers in the near future, this include some 160,000 Cambodians.
Rights activists say the case not only demonstrates Cambodia’s culture of impunity, but also its gender inequality.
Both activists were charged under a new penal code for “aggravating circumstances” in the alleged assault of two men in August.
The court’s decision set off a new round of outrage from local and international rights groups and others in the international community.
Chan Soveth, who is an investigator for the rights group Adhoc, was questioned and released by the court on Monday.
Migrant workers earn around $200 a month in Thailand, working long hours in poor conditions on construction projects, farms, fishing boats and other enterprises.
The court quietly dropped the case on Dec. 14, a move that will draw more criticism for the country’s judiciary, which is widely viewed as biased and corrupt.
Chan Soveth has been charged with abetting the escape of several suspects in a security crackdown in Kratie province earlier this year.