Cambodian opposition leader Kem Sokha has been released from house arrest after being held for more than two years on treason charges.
The charges have not been lifted, however, and he remains banned from politics or leaving Cambodia.
Mr Sokha is accused of plotting to overthrow Hun Sen, who has been in power for more than three decades.
His opposition party has been outlawed giving the government full control of the Cambodian parliament.
Mr Sokha's political partner, Sam Rainsy - who also faces wide-ranging charges in Cambodia - said that he was barred from returning from self-imposed exile in Paris on Saturday.
Why was Kem Sokha locked up?
Former political rivals Mr Sokha and Mr Rainsy founded the Cambodia National Rescue Party (CNRP) in 2012.
In the 2013 general election, they came within just seven seats of victory over the ruling Cambodian People's Party.
In September 2017, armed police raided Mr Sokha's home and he was accused of plotting to start a US-backed revolution.
This was based upon an old video where Mr Sokha was seen telling an audience in Australia that he had been receiving political advice from the US.
Hun Sen's government proceeded to outlaw the opposition, taking all 125 seats in the 2018 election.
Why has he been released?
A court ruling cites Mr Sokha's health concerns.
In a Facebook post, the opposition leader wrote: "As an innocent person who has been jailed for two years, I continue to demand that the charges against me be dropped."
Mr Rainsy, who has been living in exile since 2015, says he was barred from checking in for a flight from Paris when he attempted to return to Cambodia on 9 November.
He eventually went to Malaysia, but it is not clear whether he still plans to return to Cambodia.
He welcomed the decision on his colleague as "a step in the right direction", but told the BBC it was not enough.
"For Hun Sen's first positive gesture - attributable to increasing internal and external pressure - to be meaningful, the ludicrous 'treason' charge levied against Kem Sokha must be dropped, thus eliminating all restrictions on his freedom.
"More importantly, such dropping of the charge would open the way for a reinstatement of the banned opposition Cambodia National Rescue Party, which was dissolved in 2017 precisely on the base of this unsubstantiated 'treason' charge."
Dozens of opposition supporters in Cambodia have been rounded up in recent weeks ahead of Mr Rainsy's supposed return, which Hun Sen had described as an attempted coup.
Cambodia is under international pressure, with the European Union considering revoking its preferential trade terms due to Hun Sen's authoritarian rule.