Uganda's main opposition leader Kizza Besigye has been arrested in connection with the death of a policeman.
The officer was hit on the head by a stone when police intervened to break up a gathering of opposition supporters in the capital, Kampala.
The police said 16 people, including Mr Besigye and the city's mayor, were in jail and would be taken to court.
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.
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.
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.