Trade talks between the US and South Korea took a nasty turn as protesters clashed with police in Seoul.
The talks have faced stiff opposition from Korean farmers
About 30,000 demonstrators gathered in the city centre, some throwing missiles and trying to overturn buses. Riot police responded with water cannons.
No arrests were made and no-one was injured, authorities said.
The protests took place on a third day of talks aimed at agreeing a deal to slash tariffs and remove other trade barriers between the two countries.
Opponents in South Korea - including farmers and labour groups - say any deal will lead to cheaper US imports, while critics in the US say an agreement will threaten American jobs.
However, supporters say the deal would boost growth and strengthen ties with the US.
If an agreement is reached it would signal the biggest free trade pact involving the US since the North American Free Trade Agreement (Nafta) with Canada and Mexico was created in 1993.
South Korea is the US's seventh-biggest trading partner, and trade between the two countries totalled more than $72.5bn in 2004.
The US is South Korea's second-biggest trading partner after China, and Seoul's main source of foreign direct investment.
Negotiations between the two sides kicked off earlier this year, but little progress has since been made, despite negotiators pushing for an agreement by January 2007.
The current round of talks is expected to end on Friday.