Thousands of South Korean farmers and fishermen rallied Tuesday against a proposed free trade deal with China as the two countries started a second round of talks.

"We oppose a Korea-China free trade agreement!" they shouted at a protest outside Seoul city hall. Police said about 10,000 took part.

The protesters -- waving flags and wearing headbands with slogans such as "Stop Korea-China FTA!" -- said any deal would flood local agricultural and fisheries markets with sub-standard Chinese products.

An agreement would also "eventually threaten the health of Koreans and kill the (two) industries", they said in a statement.

The two countries launched free trade talks in May after a series of joint feasibility studies and began a second round Tuesday in the South's Jeju island.

Agriculture and fisheries are considered the most sensitive sectors for South Korea, while China categorises its manufacturing industries as sensitive.

China is South Korea's largest trading partner, with bilateral trade expected to reach $300 billion by 2015, up from $245.6 billion last year, according to Chinese data.

Seoul policymakers say such a pact would allow South Korea better to compete against Taiwan in the lucrative Chinese market after Taipei struck a trade agreement with Beijing in 2010.

Export-dependent South Korea has already signed free trade deals with the United States, the European Union, India and the Association of Southeast Asian Nations, and is expanding its commercial reach in Latin America.