Jailed Ugandan opposition figure Kizza Besigye boycotted the start of his treason trial on Monday, accusing the presiding judge of bias, his attorney said.

Besigye's months-long detention has shone a spotlight on President Yoweri Museveni's human rights record ahead of polls early next year in which Museveni, 80, is seeking re-election.

The trial of Besigye and his aide, Obed Lutale, was supposed to start on Monday after months of delays, but both defendants decided to boycott the proceedings after the judge, Emmanuel Baguma, refused to recuse himself, their attorney Eron Kiiza told Reuters.

Besigye's attorneys cited Baguma's decision to deny Besigye bail as the basis for the accusations of bias.

Judiciary spokesperson James Ereemye Mawanda said there was no justification for the bias accusations and confirmed that Baguma had refused to recuse himself. Judge Baguma could not be reached immediately.

"Besigye and Lutale took a decision never to appear before Judge Baguma," attorney Kiiza said.

"He is incapable of delivering justice with fairness and impartiality as required by the constitution and logic," Kiiza added.

A former ally and personal physician of Museveni, Besigye has stood and lost against Museveni in four elections. He has not indicated whether he wants to run again.

He was detained alongside his aide in November last year in neighbouring Kenya and returned to Uganda where both were subsequently charged with treason and other offences, initially in a military tribunal before the case was transferred to a civilian court.

He denies the charges.