US President Barack Obama and other world leaders are due to arrive in the South Korean capital on Thursday for the opening of the G20 summit.
But as Al Jazeera's Steve Chao reports from Seoul, union workers and civic groups have already begun one of the largest rallies in the run-up to the Thursday-Friday meeting.
The protesters are opposed to a long-delayed free-trade deal between South Korea and the US, which Lee Myung-bak and Barack Obama, respectively the two countries' presidents, want to finalise before the summit.
The demonstrators also want the South Korean government to improve labour rights.
For their part, authorities have tightened security in preparation for the arrival of 10,000 participants, including 32 heads of government and leaders of international organisations.
South Korea is concerned about the risk of violent anti-capitalist protests - a common feature of summits involving the world's leading economies - and also worried that 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.
Source: Al Jazeera and agencies