An Associated Press photographer reported minor scuffles between police and demonstrators making their way to a "Carnival Against Capitalism" planned by the group StopG8.
Some protesters congregated outside the offices of oil giant BP, unfurling a banner that read "Stop the G8!!"
Before the protests, riot police also swooped in on a building occupied by the anti-G-8 protesters. Video footage showed officers in climbing gear on the building's roof, tackling a protester.
Scotland Yard said 32 people have been arrested for a variety of alleged offences, including assault on police and criminal damage.
G-8 leaders will meet in Lough Erne, Northern Ireland, on June 17-18. The group is made up of the world's eight richest economies: the U.S., Britain, Canada, France, Germany, Italy, Japan and Russia.
The summit will be on the agenda Wednesday when U.K. Foreign Secretary William Hague travels to Washington to meet U.S. Secretary of State John Kerry. They also are expected to discuss Syria and the Middle East peace process.