Human rights groups condemned a decision by the Zimbabwean High Court on Wednesday to uphold a lower court's ruling denying bail to an opposition party leader and 78 activists, who were arrested last month.

Jameson Timba, who took over as party leader at Citizens Coalition for Change (CCC) after Nelson Chamisa quit in January, was arrested along with other party youth in Harare for holding an unauthorised gathering.

They were charged with gathering with intent to promote public violence and denied bail by a Magistrates Court.

Amnesty International late on Wednesday demanded their immediate release and described their continued imprisonment as "lengthy" and "baseless".

"Their arbitrary, month-long detention is an outrage which must end now," said Khanyo Farise, Amnesty's deputy regional director for East and Southern Africa.

Their lawyers have alleged police assault while others sustained injuries.

A High Court judge on Wednesday said he would not interfere with the decision of the lower court, arguing that the activists were not "randomly" assembled on the day of their arrest.

CCC said the decision by the court was "unjust".

"Despite bail being a constitutional right, the regime is blatantly denying innocent citizens this fundamental right," CCC said.

The Zimbabwe Lawyers for Human Rights, a local rights group which is providing legal support, said it will continue to explore "legal options to assert their right to liberty".