Around 20,000 protesters have chanted anti-globalisation slogans and marched through downtown Seoul streets in a rally against this week's Group of 20 summit to be held in the Korean capital.
No violence was initially reported at the protest, organised by the Korean Confederation of Trade Unions, but some protesters engaged in minor scuffles with riot police trying to prevent them from marching through nearby Seoul streets later on Sunday.
According to reports, police fired pepper spray at part of the crowd. It was the first major anti-globalisation demonstration before South Korea hosts a gathering of leaders from the G20 advanced and emerging economies on Thursday and Friday.
The protesters on sang, danced and waved signboards with signs reading "We oppose the G20" during the rally at a large square near Seoul City Hall.
Around 9,000 riot police and numerous police buses encircled the rally site to keep order.
Activists plan to stage more, though smaller, rallies every day until the summit begins on Thursday, according to a labour activist.
South Korean police and troops are on heightened alert to prevent any attempt to sabotage the summit. Rival North Korea has a history of staging provocations when world attention is focused on South Korea.