The protesters, including transport and metal workers, belong to the Korean Confederation of Trade Unions, an umbrella labour group.
They also announced a general strike on Wednesday to push their demands.
''No FTA'', the workers shouted. They also urged the government to enact laws to protect contract workers.
The protest is one of a long line of demonstrations by workers and farmers who fear the pact would destroy their livelihoods.
South Korea hosted a fourth round of talks on the trade deal in late October. That event was marred by violent protests by farm activists, fishermen and unionists.
The United States said at the end of those talks that South Korea's latest offer to open its heavily protected agricultural market did not go far enough.
Differences still exist between both sides on opening up trade in farm goods, automobiles, textiles and pharmaceuticals.
Negotiations will resume in December in the United States and are expected to stretch into next year.
REUTERS AKJ PM1459