Millions of Cambodians will head to the ballot box on Sunday for local polls across more than 1,600 communes -- an early litmus test for next year's crunch general elections.
The impoverished Southeast Asian kingdom has been run for more than 32 years by strongman prime minister Hun Sen, one of the world's longest serving leaders.
But in 2013 his ruling party suffered a surprise setback when the opposition Cambodia National Rescue Party (CNRP) made huge gains and nearly won.
In a report titled "Courts of Injustice", the group said at least 27 Cambodian human rights defenders and political activists are currently behind bars on trumped-up charges.
More From This Section
Hundreds of others are subject to criminal proceedings "as part of a concerted attempt" to crush any public criticism.
As a result a large proportion of the political opposition and human rights community "live under the threat of immediate imprisonment", the report said.
Champa Patel, Amnesty's director in the region, said Hun Sen's government has paid "much lip-service" to the judiciary's independence.
Sok Eysan, a spokesman for ruling Cambodian People's Party (CPP), denied the report's allegations and said it was peddling a "Cold War ideology".
Amnesty has always viewed the government as the "enemy", he said.
"They have been attacking us and painting colours on the ruling party since the beginning."
Sunday's election is a bellwether for opposition efforts to unseat Hun Sen after three decades.
Sebastian Strangio, an expert on Cambodian politics, told AFP Hun Sen's party "risks losing control of a lower level of government that they have controlled since the fall of the Khmer Rouge in 1979".
But last week he said he would attend a rally and parade on Friday -- something analysts said indicated his nervousness.
Hun Sen and his defenders say the 64-year-old has brought much needed stability and growth to Cambodia after decades of crippling civil war and genocide.
Detractors say he and a coterie of political allies have amassed huge riches while presiding over an endemic culture of corruption.
His popularity is especially low among the young who make up a huge chunk of Cambodia's population and voted in droves for the opposition in 2013.