Activists gathered in Kenya's capital of Nairobi on Thursday to demand the release of Ugandan opposition leader Kizza Besigye, a day after he collapsed in court in Kampala.

The activists described Besigye's condition as critical and accused Ugandan authorities of frustrating due process in the opposition leader's treason case.

"Eyewitness, a legal account described a shocking, frail state of deterioration, with Dr. Besigye exhibiting yellowing eyes, labored breathing and gasping for air. He is currently fighting for his life under hostile military guard in the ICU at Mulago National Referral Hospital," said Mwanasiti Athman, an activist.

They also called for the release of Erias Lukwago, Besigye's lawyer who is being held in Luzira Prison with charges linked to the case.

"We demand, number one, immediate release of Dr. Kizza Besigye on humanitarian grounds. Court-ordered authorities must immediately grant unconditional bail to Dr. Besigye to permit urgent specialised medical treatment," said Michael Ngige, another activist.