About 9,000 people, according to police, gathered downtown for a candlelight rally in front of city hall before marching through nearby streets guarded by anti-riot personnel.
The demonstration was one of the largest in recent weeks against an agreement with Washington to reopen South Korea to American beef, banned for most of the past 4 1/2 years over fears of mad cow disease.
Earlier Thursday, Agriculture Minister Chung Woon-chun said the government would implement the accord, which was announced on April 18 just hours ahead of a summit between the presidents of the two countries in the United States.
But almost daily protests this month have hamstrung plans to take final administrative and legal steps — such as the announcement of revised quarantine regulations — necessary to implement the deal.
Protesters claim the government is ignoring their worries about the safety of U.S. beef. They also accuse President Lee Myung-bak of making too many concessions on beef to gain U.S. congressional approval for a broader bilateral free trade agreement struck last year but that awaits legislative approval in both countries.
Members of the U.S. Congress from cattle-producing states had said the free trade deal has no chance of passing unless South Korea ended the beef ban.
Some of the mostly peaceful rallies have drawn as many as 10,000 protesters. Tensions only flared this week after the government instructed police to take a harder line.
Police have detained more than 200 protesters, with many subsequently released. There were no reports of clashes or arrests Thursday night.
The demonstrations have become a major headache for Lee, who took office just three months ago on a wave of popularity.
Last week, hoping to assuage anger, he apologized for not sufficiently consulting the public on the beef issue.
Despite the protests, not all South Koreans oppose the return of U.S. beef, citing expectations it will be cheaper than pricey domestic varieties.
South Korea suspended imports after the first U.S. case of mad cow disease appeared in December 2003 in a Canadian-born cow in Washington state. Two subsequent cases were also discovered in the U.S.
Before the ban, South Korea was the third-largest overseas market for U.S. beef. A previous deal allowed restricted imports to reach South Korean consumers last year, though they were quickly suspended after banned substances such as bones were found in shipments.
Repeated statements by both governments that American beef poses no health risk have run up against persistent fears fanned by media reports and Internet rumors as well as the view that Lee has behaved arrogantly.
"I'm against U.S. beef because they will export trash," said protester Lee Seon-ok, voicing a widely held opinion among demonstrators that U.S. meatpackers plan to ship beef from older cows that are believed to have a higher risk of harboring the brain-wasting cattle disease.
"The Americans don't care about our health," she said.
Scientists believe mad cow disease, or bovine spongiform encephalopathy, spreads when farmers feed cattle recycled meat and bones from infected animals. The U.S. banned recycled feeds in 1997.
In humans, eating meat products contaminated with the cattle disease is linked to variant Creutzfeldt-Jakob disease, a rare and fatal malady.
Some 5,300 tons of U.S. beef, shipped earlier but held up in customs and quarantine storage facilities, will begin undergoing inspections early next week before being put on the market, the government said.
The new quarantine rules represent a significant easing of previous ones, which banned the import of beef attached to bones or from older cattle considered to have a potentially higher risk of mad cow disease.