"We'll protest in a lawful way and in a creative and popular way," Park Seok-woon, of the Korean Alliance Against the Korea-U.S. FTA, told reporters at the Seoul Foreign Correspondents' Club.
The group, consisting of various labor, farm and social groups, said it expects about 40-50 members to go to Washington for the protest next week.
They plan a weeklong series of demonstrations, rallies and press conferences to gain support against the agreement, which members say "will threaten the livelihood of all" South Koreans.
South Korea and the United States announced in February they would launch free trade talks that, if successful, would be the biggest such accord for Washington since the North American Free Trade Agreement in 1993.
Formal negotiations are to begin Monday in Washington.
The two sides want an agreement ratified by their lawmakers before the Bush Administration's "fast track" trade authority, which allows it to negotiate an agreement and submit it to Congress for a simple yea-or-nay vote without amendments, runs out in mid-2007.
About 1,000 protesters - many of them South Koreans - were rounded up in Hong Kong in December after anti-globalization protests during a World Trade Organization meeting turned violent.
Protesters rammed police lines with metal barricades and struck officers with sticks. A total of 14 demonstrators - including some South Koreans - were arrested and charged, though there were no convictions.
South Korean Foreign Minister Ban Ki-moon issued a public appeal last month urging groups opposed to the agreement to abandon plans to demonstrate in the United States.
There are several versions of the recent assassination of the most prominent Iranian nuclear scientist and high-ranking officer of the Islamic Revolutionary Guard Corps, Mohsen Fakhrizadeh