Human Rights Watch says bail should be granted to Ugandan opposition leader Kizza Besigye and that he be given a fair trial.
Mr. Besigye and 22 others were arrested Monday on charges of trying to topple the government. He also faces a separate charge of rape.
If found guilty, Mr. Besigye could face the death penalty.
Prosecutors say Mr. Besigye, leader of the opposition Forum for Democratic Change, is involved with two Ugandan rebel groups (the People's Redemption Army and the Lord's Resistance Army). Mr. Besigye has denied those charges in the past.
Mr. Besigye ran against President Yoweri Museveni in 2001 before fleeing the country, saying his life was in danger.
He returned to Uganda just three weeks ago. He has been widely expected to be President Museveni's most serious challenger in next year's elections.
Shortly after his arrest, police fired rubber bullets, tear gas and water cannons on rioting opposition supporters who had gathered in Kampala, Uganda's capital.