"He was released on bail and he is under monitoring by the court," government spokesman Phay Siphan told Reuters on Sunday.

Kem Sokha's daughter, Kem Monovithya, said her 65-year-old father has been placed under house arrest, is in poor health and needs medical attention.

"He has high blood sugar and he needs a long overdue surgery on his left shoulder," she said.

Long-serving Prime Minister Hun Sen had been under pressure to release Kem Sokha following a July general election won by his Cambodian People's Party (CPP).

Kem Sokha, leader of the now-dissolved Cambodia National Rescue Party (CNRP), was arrested and accused of treason last September as part of a government-led crackdown against critics, including the CNRP.

He had been in pre-trial detention since then, held at a remote prison near Cambodia's border with Vietnam.

Cambodia's Supreme Court dissolved the CNRP last year at the request of Hun Sen's government in a ruling that supporters of the CNRP said was politically motivated.

Former CNRP president Eng Chhai Eang wrote on his Facebook page that Kem Sokha was released from prison and "has arrived at his home". CNRP deputy president Mu Sochua said Kem Sokha had arrived home "a few hours ago".

"We don't know much more," she told Reuters.

Reuters was not able to reach Kem Sokha immediately for comment.

Hun Sen's CPP won all 125 parliamentary seats in the July vote, which the United Nations and some Western countries have said was flawed because of the lack of a credible opposition, among other factors.

The CNRP, which had 118 of its members banned from politics for five years following the party's dissolution, has called the July election a farce.

Observers say Hun Sen has been relaxing his stance against critics since the July win, which secured him another five years in power.

Fourteen government critics were freed from jail last month in a move some saw as meant to appease foreign criticism of the election.

Australian Associated Press