Cambodia's opposition leader Kem Sokha has been in jail for a year is suffering ill health.
Sokha's release, little more than a month after Hun Sen claimed victory in what was widely derided as a sham election, comes amid growing international condemnation of the Cambodian regime.
The former leader of the Cambodian National Rescue Party was arrested and accused of treason last September by the government amid a crackdown on what was then the country's main opposition party.
Hun Sen's Cambodian People's Party won all 125 seats in July's national election, which was held after 118 members of the now-disbanded CNRP were banned from politics for five years, and after many other high profile opposition figures fled into exile.
Cambodian government spokesman Phay Siphan confirmed to Reuters on Monday that "he was released on bail and he is under monitoring by the court".
Sokha's daughter, Kem Monovithya, who is herself a senior figure in the CNRP, said her 65-year-old father had been placed under house arrest, was in poor health and needed medical attention.
Cambodia's Prime Minister Hun Sen at the National Assembly, in Phnom Penh, Cambodia, last week.
It is not clear what legal conditions have been attached to Sokha's release, and the former opposition leader has not made a public statement since his release.
Sydney University Cambodia expert Jean Bogais said the decision to release Sokha was a cynical ploy and fitted within the pattern of Hun Sen manipulating and appeasing the international community for much of his 33-year rule.
"Having won the election when there was no opposition, now he is trying to show he can compromise. But this is very much an empty gesture, he said.
"Hun Sen has succeeded in what he wanted to achieve [winning the election], now he is just making a gesture."
The strongman leader was likely concerned about threats from the United States and the European Union to extend sanctions and visa bans on top officials within the government party, Associate Professor Bogais said.
He said the decision to release Sokha, which follows the post-election release of other jailed politicians, activists and journalists, was likely also a gesture to the ASEAN group of nations.
The Australian government has been comparatively muted of Hun Sen's effective moves to make Cambodia a one party state.
But ASEAN Parliamentarians for Human Rights chair Charles Santiago, a Malaysian MP, said the sudden release of Sokha highlighted the political nature of the charges against him.
"His release, only after Prime Minister Hun Sen has shored up control of the parliament and country through a fake election, portrays a government seeking to legitimise the illegitimate," he said.
"Kem Sokha should be released unconditionally and all charges be dropped. We welcome Kem Sokha's release from prison detention, but it is far from all that is required."
Human Rights Watch's deputy Asia director Phil Robertson said Sokha's failing health likely also played a part in his release, as the Hun Sen regime wanted to avoid a possible public relations disaster if something had happened while he was still being held in a remote prison.
“The only way justice will be served is when the charges against Kem Sokha are dropped without conditions, the CNRP dissolution is reversed, and the political rights of Sokha and the other 117 CNRP senior executives are fully restored," Robertson said.
"Despite the excitement connected to today’s release, we’re still a long way from a meaningful restoration of democracy and respect for human rights in Cambodia.”