Uganda's main opposition leader Kizza Besigye was arrested after the death of a policeman during clashes in Kampala.
The officer was hit on the head by a stone when police intervened to break up a gathering of opposition supporters in the capital.
Mr Besigye was later charged with "unlawful assembly" and is now house arrest, his party told the BBC.
Mr Besigye lost presidential elections last year and has been arrested several times since in similar disputes.
The BBC's Ignatius Bahizi in Kampala says the trouble began as opposition leaders touring development projects in the city were joined by their supporters in what became a procession through the streets.
The police said it was an illegal gathering and fired tear gas to disperse the crowd. Sixteen people, including the city's mayor, were arrested.
The senior officer who was hit by a stone died on the way to hospital, police spokesperson Judith Nabakooba said.
"Wanton violence against police officers on duty shall be sternly dealt with to the full extent of the law," she told reporters.
Anne Mugisha, from Mr Besigye's Forum for Democratic Change party, told the BBC that she feared the death of the police officer would lead to a clamp down of future protests.
"It is has now become illegal.... for Dr Kizza Besgiye to walk in downtown Kampala, because he draws crowds," she told the BBC's World Today programme.
"And as you know, this government has been jittery ever since the Arab Uprising, that the opposition is planning to go and occupy a square in the city and cause international attention which will lead to regime change."
Last year there was a wave of protests following President Yoweri Museveni's re-election in February - a poll denounced as fraudulent by opposition groups.
Mr Besigye was Mr Museveni's personal doctor before the pair fell out.