A Ugandan judge refused to grant bail on Friday to veteran opposition figure Kizza Besigye, who has been in jail for nearly nine months on treason charges.

The case has raised concerns among government critics, including opposition leader Bobi Wine and rights groups, about a crackdown ahead of Uganda's national election early next year in which President Yoweri Museveni, 80, is seeking re-election.

The government denies targeting opposition figures and says all those who have been detained have committed crimes.

A former ally and personal physician of Museveni, Besigye has stood and lost against Museveni in four elections. He has not said whether he is running again.

Besigye, who denies any wrongdoing, was forcefully returned to Uganda from neighbouring Kenya in November last year, and initially charged in a military tribunal, before his case was transferred to a civilian court.

His lawyers argued he should be automatically released on bail because he has spent more than 180 days in jail without his trial starting.

Judge Emmanuel Baguma said the 180-day maximum period before mandatory bail is granted only began when he was remanded in the civilian court on February 21, which means he falls short by 12 days in order to secure bail.