Cambodia’s National Assembly has approved a bill making it a crime to deny that atrocities were committed by the genocidal 1970s communist Khmer Rouge regime. Critics say the law will be used as a weapon against the political opposition.

The assembly passed the bill unanimously in the absence of opposition lawmakers, who were expelled from the legislature this week. A committee controlled by the ruling Cambodian People’s Party said the lawmakers must relinquish their seats under the law because they had joined a new party to contest this July’s general election.

Prime Minister Hun Sen called for the law after a leading opposition lawmaker reportedly suggested that some of the evidence of Khmer Rouge atrocities was fabricated by Vietnam, whose army invaded to oust the Khmer Rouge in 1979.