PHNOM PENH, Cambodia — Cambodia’s imprisoned opposition leader was freed on bail Sunday night after spending a year locked up on charges of treason widely seen as designed to neutralize his political power during a crucial election year.

The daughter of Kem Sokha, Kem Monovithya, confirmed that her father had been quietly released from the remote border prison where he was being held and then driven to his Phnom Penh home under guard. As a condition of his release, he will not be able to leave the city block surrounding his house, she said.

His bail was granted on grounds of ill health, according to a statement released by a Cambodian court. After having been held in isolation and with scant access to medical care for a year, Mr. Kem Sokha is suffering from diabetes and high blood pressure and has a serious shoulder issue that requires medical attention, his family has said.

The opposition leader was arrested at midnight on Sept. 3, 2017, and subsequently accused of conspiring with the United States in a plot to bring down Cambodia’s government. Mr. Kem Sokha has maintained his innocence and said he was only trying to take power through legal means, by winning elections.