Clashes broke out after more than 15,000 protesters headed toward the presidential office, the news agency said.

The demonstrators used ropes to pull down police buses that had been parked around the presidential office to block the marchers from entering.

Many carried steel pipes and hurled rocks at officers. Police fired back with water cannons and fire extinguishers.

RoK people have protested regularly since April when the government announced it would resume importing beef from the US after a five-year ban. That ban was instituted over a case of mad cow disease in 2003.

The same day, US Secretary of State, Condoleezza Rice, during a visit to Seoul as part of an Asian tour, emphasised that American beef is safe and that her government will continue efforts to win consumer confidence.

Scientists believe mad cow disease spreads when farmers feed cattle recycled meat and bones from infected animals. The US banned recycled feeds in 1997.

Until the 2003 ban, the RoK was the third-largest market for US beef exporters. The US beef industry has lost up to US$4 billion since the market closed.