Tens of thousands of South Koreans, most of them members of trade unions and farmers, staged a rally on Sunday against a free trade deal with the United States, vowing to block what they called a “pernicious” pact. South Korean President Lee Myung-bak’s government has been under pressure to ratify the agreement after the US Congress approved it last month and President Barack Obama signed it into law. The deal is the biggest US trade pact since the North America Free Trade Agreement went into force in 1994. But South Korean opposition party members have been trying to block it in parliament, some complaining that the deal gives US automakers a major inroad into the South Korean market. Other critics have been taking their complaints to the streets. About 20,000 people, including teachers, students and public servants gathered in Seoul to demand that the government declare the Free Trade Agreement (FTA) null and void. It was the biggest protest in the capital this year. “Let’s block the FTA, which is unpatriotic and pernicious to the nation, at any cost,” protesters chanted during the rally at a plaza in front of Seoul City Hall. Some studies found that the deal could boost the allies’ $67 billion two-way trade by as much as a quarter. Despite the complaints that it gives the US auto industry major access to the South Korean market, Korean automakers stand to gain with swifter and greater access to the US market. US farmers are expected to be one of the biggest winners under the agreement, with more than $1.8 billion a year in increased exports to South Korea. But that’s a worry for many South Korean farmers.