Besigye, 69, is a leading opponent of President Yoweri Museveni — in power for nearly 40 years — having unsuccessfully challenged him in four elections.

He was abducted in Kenya in November and returned to Uganda where he faces the death penalty for treason, charges widely condemned by international rights groups.

His trial was due to start on Monday, but he and his co-accused, Obeid Lutale, “refused to appear in court… because they believe they will not get a fair hearing,” one of his lawyers, Erias Lukwago, told AFP.

“The two have applied to the court to have Judge Emmanuel Baguma recuse himself from the case but he has refused and they have taken a decision not to appear before him until another judge is appointed,” he added.

The defendants say the judge is biased because he refused to grant them bail.

The legal limit for detention without trial in Uganda is six months, but the judge last month ruled that the clock only started in February when their charges were read, rather than the moment when Besigye was abducted and moved to a military prison.

Their lawyer earlier labelled the ruling “absurd”.

Rights groups have linked Besigye’s abduction and trial to January’s election when Museveni, 80, will seek another term in office.

In June, Besigye’s wife, UNAIDS executive director Winnie Byanyima, said he was being detained “for political reasons” under “inhumane” conditions.

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

However, Museveni later signed a new law reinstating military trials for civilians under “exceptional circumstances”.

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© Agence France-Presse