SEOUL, Nov 11: South Korean demonstrators and riot police clashed in downtown Seoul on Sunday as tens of thousands of anti-American protesters were blocked from marching to the US embassy after an unauthorised rally.

The violence erupted when 20,000 riot police confronted the same number of demonstrators rallying against a free-trade pact with the United States and South Korean troops joining US-led forces in Iraq and Afghanistan.

Having been blocked from marching to the US embassy, hundreds of angry protesters smashed the windows of police buses, wielded wooden sticks and hurled rocks at riot police.

Police reacted with batons, shields and water cannons to stifle the rioters.

Both sides exchanged kicks and punches, but no serious injuries were immediately reported.

Police rounded up dozens of violent protesters, a Seoul metropolitan police agency spokesman said, adding they were still compiling arrest figures.

The agency said it sent 23,000 police and 600 buses to stop the massive demonstration in the capital that had been banned by the government.

Civic and labour organisations had expected about 50,000 people to turn up for the rally, which also called for better working conditions for temporary workers, but police said the actual turnout was 20,000.

Demonstrators wanted to rally at the City Hall plaza and march a block towards the US embassy, but police blocked them from reaching the area.

A hardcore 2,000 demonstrators occupied parts of the 16-lane main boulevard in Seoul to hold a candle-lit vigil in the evening, though most returned home in the afternoon.

The ministers of home, justice, transportation and labour issued a joint statement Saturday banning the rally, which comes at a politically sensitive time ahead of the December presidential election.

It warned that police would deal “sternly” with any illegal or violent acts.

Yonhap news agency said police had blocked farmers from being bussed to Seoul from towns and villages.

Farmers and activists have sometimes violently protested the trade deal with the US, which now must be ratified by lawmakers, but most opinion polls show a majority of South Koreans favour it.

The United States and South Korea signed the free trade pact in June following 10 months of intense negotiations, but little progress has been reported on either side to get the deal ratified.—AFP