Rong Chhun told cheering supporters outside the prison that he would continue his activities and urged all Cambodians to fight for freedom and human rights.
He said he was convicted unjustly and that the court should drop all charges against him and restore his rights in addition to releasing him early.
Also ordered released were two co-defendants, Ton Nimol and Sar Kanika, who were found guilty of incitement to commit a felony. They were arrested in August 2020 while demonstrating for the release of Rong Chhun, and each was sentenced to 20 months in prison in the same trial.
Two other activists in an unrelated case were also ordered to be released.
Labor leaders such as Rong Chhun hold significant political influence in Cambodia because they represent the vast number of workers in the textile industry, which is a major export earner. The major unions have historically aligned themselves with the political opposition to Prime Minister Hun Sen.
Rong Chhun served on the national election committee of the opposition Cambodia National Rescue Party before it was dissolved by court order in 2017, ahead of the 2018 general election.
The party's dissolution was generally seen as intended to ensure victory for Hun Sen’s ruling Cambodian People’s Party. Hun Sen has been in power for 36 years and has often been accused of heading an authoritarian regime.
In a separate case, rights groups called for the immediate release of two other opposition CNRP activists who were deported back to Cambodia from Thailand earlier in the week.
Voeun Veasna and Voeung Samnang were both wanted in Cambodia for charges related to online postings critical of the government.
The regional advocacy group ASEAN Parliamentarians for Human Rights called for their immediate release and criticized Thailand, saying it was “unacceptable to return a refugee to a country where they are likely to face persecution.”
Human Rights Watch said Thailand's return of the two men “shows a blatant disregard for fundamental refugee protection principles.”
“The Thai government’s actions make it complicit in the Cambodian government’s persecution of its political opponents, which appears to extend beyond Cambodia’s borders," said Bill Frelick, the organization's refugee and migrants director.
Thai immigration authorities could not immediately be reached for comment.
Rising reported from Bangkok.