Some demonstrators were carried away with head wounds, splashed with blood, after more than 100 police blocked a march on Kampala city centre and charged with batons.
Witnesses said one police officer suffered a head injury when the demonstrators threw stones after the march was halted.
"This is nothing new. This is what we have been living through all this time. This is a police force that does not respect the rights of citizens at all," opposition leader Kizza Besigye said.
Police said the protesters knew they were not allowed to march out of a suburb where the demonstration was authorised.
Protesters said the Commonwealth, which began a three-day summit yesterday, had ignored human rights abuses by the government of President Yoweri Museveni, which diplomats say ruthlessly suppresses opposition in Uganda.
Elizabeth Evenson, of US-based Human Rights Watch, condemned the police action.
"Using force to keep opposition supporters out of sight of the Commonwealth summit reflects continuing limitations on political freedoms. Commonwealth leaders should take President Museveni to task," she said.
The demonstrators earlier waved banners criticising Queen Elizabeth, who opened the summit. "Queen, you are the head of our problems," one of them read.