The wife of imprisoned Ugandan opposition leader Kizza Besigye said on Thursday he was being held under inhumane conditions and condemned his continued detention.

Besigye, 69, is a leading opponent of President Yoweri Museveni – in power for nearly 40 years – whom he has unsuccessfully challenged in four elections.

He was abducted in Kenya in November and faces the death penalty for treason, charges that have been widely condemned by international rights groups.

UNAIDS Executive Director Winnie Byanyima said her husband's treatment was "inhumane", describing how he was forced to sleep on a bedbug-infested mattress, had been isolated from other prisoners, and was constantly accompanied by guards.

'Attacked by bedbugs'

"Dr. Besigye has been in the tiny cell and now he has been attacked by bedbugs. Bedbugs are biting him day and night," she said.

"What they (authorities) are doing to him is violating his rights, his human rights, and the laws of Uganda," Byanyima told reporters in Kampala, after visiting Besigye in prison.

"Every rule in Uganda seems to be broken when it comes to Dr. Kizza Besigye."

He appeared visibly frail during a court appearance in February, following a brief hospitalisation.

Byanyima says husband detained 'for political reasons'

"I want to emphasise that Dr. Besigye is not there for anything," she said, expressing anger over the situation and insisting that her husband was being detained "for political reasons."

"He is being criminalised because he has challenged, he has put himself forward as a candidate in elections," she said.

Rights groups have said that Besigye's abduction and trial for treason are linked to the election in January, when Museveni, 80, will seek to extend his term.

His case was initially tried in a court martial but later moved to a civilian court after the Supreme Court ruled it was unconstitutional to try civilians in military courts.

In jail for over six months

But on Monday, Museveni signed a new law reinstating military trials for civilians under "exceptional circumstances", including the "unlawful possession of arms, ammunition or equipment" – one of the charges Besigye is also facing.

Besigye has been in jail for more than the six-month legal limit for detention without trial.

AFP has contacted the Ugandan prison service for comment, but had not received a response by publication.