SEOUL Tens of thousands of people took to the streets of Seoul on Sunday in a largely peaceful protest against this week's G20 summit as the city went on heightened alert for the meeting of world leaders.
Authorities ramped up security at the weekend in preparation for the arrival of 10,000 participants, including 32 heads of government and leaders of international organizations, for the summit on Thursday and Friday.
South Korea is concerned about the risk of violent anti-capitalist protests -- a common feature of summits involving the world's leading economies -- and anxious its rival North Korea may try to stage an incident to embarrass it.
Security forces have been put on high alert, anti-aircraft missiles are at the ready, shipping and air routes are under heightened surveillance and airport screening has increased.
South Korea's armed forces are on a "Level 3" alert, the highest level, and some 50,000 police -- more than one-third of the national force -- launched increased patrols from Saturday.
Sunday's protest, involving between 20,000 and 40,000 protesters, was seen as a test of the mood of demonstrators, and how the police would respond to trouble.
A Reuters correspondent said the atmosphere was generally restrained, although police used pepper spray to stop protesters from marching away from the main rally site. They were blocked by thousands of police, and trucks equipped with water cannon and police buses. At least 10 people were detained.
Protesters were also opposed to a long-delayed free trade deal between Seoul and Washington, which leaders Lee Myung-bak and Barack Obama want to finalize before the summit. Demonstrators also urged the South Korean government to improve labor rights.
A mass rally will take place on the first day of the summit, though its exact location is unknown, as authorities have set up a 2 km (1.25 mile) security zone around the main summit venue.
(Reporting by Lee Jaewon; Writing by Jeremy Laurence; Editing by Mark Trevelyan)