Kizza Besigye appeared in court on Tuesday, a day after his arrest. Elsewhere in Kampala, his supporters were ransacking businesses, burning tyres and throwing stones and other objects at security forces in the central business district.
Private radio station Central Broadcasting Service reported that police shot dead at least one person who was trying to break into a shop.
Police spokesman Asuman Mugenyi, however, said the man died from a gunshot wound he suffered when a guard from a private security firm fired his weapon to stop him from breaking into a shop.
The dead man was among six people who were hospitalised for treatment of injuries they suffered during the protests. Police arrested 57 people during the riots, he said.
At least three ambulances were deployed to take the injured to hospital for treatment. About a dozen Red Cross workers were also on the scene.
Supporters, some shouting condemnation of President Yoweri Museveni and the proceedings, foreign diplomats and opposition politicians attended Besigye's court hearing.
A district magistrate ruled that prosecutors have enough evidence to back charges of treason - which carries the death penalty, concealment of treason and rape against Uganda's main opposition leader.
Museveni denied opposition charges that Besigye was charged
in an effort to eliminate a credible opponent from next year's presidential elections.
Besigye is accused of recruiting
and funding armed rebels
"Besigye has to prove his innocence because he is charged before the courts of law," Museveni told a conference of his ruling National Resistance Movement.
Besigye, who was greeted by huge crowds when he returned from exile last month and has mounted the strongest challenge to Museveni's 19-year rule, is accused of recruiting, funding and arming rebels with the help of neighbouring Rwanda, Congo and Sudan.
Besigye has denied past accusations from the government that he led the People's Redemption Army and had links with separate rebels of the Lord's Resistance Army.
The People's Redemption Army is described as a group of armed Ugandan dissidents based in the east of neighbouring Congo. Those insurgents have never attacked Uganda's territory or interests.
The Lord's Resistance Army is made up of the remnants of a northern uprising that began after Museveni, who like Besigye is a southerner, first took power. The rebels have declared they want to replace Museveni's government with one guided by the Ten Commandants.
Besigye was once close to Museveni and was the latter's personal physician during a five-year insurgency that Museveni led before coming to power in 1986.
Besigye finished second in 2001 presidential elections. After the elections, he fled Uganda saying he feared for his life and because Museveni had threatened him with arrest.
Critics say Museveni wants to
remain president for life
Museveni had been hailed as a reformist in a country that suffered the brutal tyranny of Idi Amin in the 1970s and 1980s.
But his progressive credentials have been called into questions amid what his critics see as signs he wants to remain president for life.