Tens of thousands of South Korean farmers and workers rallied in the capital Sunday, demanding that a free trade agreement with the U.S. be scrapped.

"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.

The protesters occupied a 16-lane road and chanted slogans against the free trade deal but no major clash with riot police, who wore helmets and carried shields, was reported.

Organizers claimed some 50,000 people participated in the protest while police estimated there were 20,000 demonstrators.

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 industries and services.

The proposed agreement must be endorsed by legislatures in both capitals before it goes into effect. The South Korean government submitted the proposal to parliament in September for approval.

The deal is the largest 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 certain constituencies, such as farmers in South Korea and labor groups in the U.S., have opposed it.

Although the deal excluded rice — a key 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 attended the rally, adding that the agriculture industry will collapse.