Ugandan police have fired tear gas and water cannon filled with a pink dye to break up an opposition vigil near the capital, Kampala.
Several hundred opposition supporters gathered for a "light a candle" ceremony to mourn at least nine people killed during protests in April.
Police said the meeting was illegal and could cause violence.
The opposition has vowed to step up protests against President Yoweri Museveni's government.
The BBC's Joshua Mmali in Kampala says police fired water cannon and tear gas to prevent the vigil from taking place in the city's Kirera suburb.
The parliamentary leader of the main opposition Forum for Democratic Change (FDC) party, Nandala Mafabi, was forced to leave after being drenched by the water cannon, our correspondent says.
'Preventing the worst'
FDC Deputy Foreign Secretary Anne Mugisha said police had criminalised the event.
"Police deployed early and turned Kireka town into a battlefield. They were determined to fight and as usual they found an excuse to brutalise peaceful activists - but we are not deterred a bit," she told Reuters news agency.
Police spokesman Ibn Senkumbi said the security forces intervened because they did not want violence to erupt.
"We didn't allow the crowd to grow big because we wanted to prevent the worst from happening. We were not opposed to the rally but they refused to listen to us on the venue where they should stage it," Mr Senkumbi said.
Mr Museveni - who has been in power since 1986 - has accused the opposition of plotting an Egypt-style uprising after failing to beat him in elections in February.
In April the opposition organised "walk to work" protests against the rising cost of food and fuel in Uganda.
The opposition says security forces were responsible for the deaths of at least nine people during the protests.
FDC leader Kizza Besigye was beaten up, arrested and charged - among others - with disobeying lawful orders and inciting violence.
Last week a court dropped the charges.