Thousands of South Korean farmers and workers clashed with riot police Sunday at a massive rally against a free trade agreement with the United States.

The clash began when the protesters tried to break through a barricade blocking the road to the U.S. Embassy. Police fired 12 water cannons and sprayed fire extinguishers to halt the protesters, a police official said on condition of anonymity, citing policy.

Some protesters swung sticks and threw stones at the riot police and tried to turn over police buses set up to block the protest in central Seoul.

Police said they arrested about 100 protesters and that more than 10 riot police were injured. Organizers said about 50 demonstrators were hurt, mostly with head injuries.

Organizers said 50,000 people took part in the protest, occupying a 16-lane road and chanting slogans against the free trade deal, while police estimated there were 20,000 demonstrators.

"We will thwart (the passage) of the FTA as it would deepen the polarization of our society," said Woo Moon-sook, a spokeswoman for the Korean Confederation of Trade Unions, one of organizers of the rally near Seoul City Hall.

Trade chiefs from the two countries signed the accord in June, concluding months of tough negotiations to lower or eliminate tariffs and other trade barriers on a wide range of products and services.

The agreement must be endorsed by the legislatures of both countries before it goes into effect. The South Korean government submitted the pact to the National Assembly in September for approval.

South Korean President Roh Moo-hyun said Sunday that his country could be left behind in the global economy without the free trade deal, saying it could give South Korea a chance to move forward.

South Korea "needs the FTA" with the U.S., Roh said in an interview with KTV, a government-run TV channel, according to his office.

The deal is the largest free trade agreement for the U.S. since the North American Free Trade Agreement more than a decade ago, and the largest ever for South Korea.

Both sides said the accord will boost growth, but some groups, such as farmers in South Korea and labor groups in the U.S., have opposed it.

Although the deal excludes rice — a key South Korean crop — farmers are worried that other kinds of U.S. agricultural produce will pour into the country, threatening their livelihoods.

"Farmers would be the biggest victim of the free trade deal," said Lee Young-soo, a farmer who took part in the rally.