Farmers said on Thursday that a free-trade pact would destroy their livelihoods and demanded a compensation package before starting talks with Washington.
They heckled a speaker at the government-backed public hearing and unfurled banners saying the trade deal would kill farmers.
Some protesters stormed the stage, preventing any further discussions, and stood their ground even after security guards tried to restore order.
Calling off the hearing
South Korean trade ministry officials decided to call off the hearing and said they would not schedule another. Economic officials planned to meet on Thursday and were expected to announce the start of talks with the US.
Lee Sung-hoon, the director for North American trade at the ministry of foreign affairs and trade, told reporters: "As far as we are concerned, a hearing has been held."
Public hearings on free-trade deals are recommended under a presidential decree but are not legally required, another trade ministry official said.
South Korean farmers and activists have held several violent protests against measures to open up the agriculture market.
They were also at the forefront of violent protests at a World Trade Organisation meeting in December in Hong Kong.
"As far as we are concerned, a hearing has been held"
Lee Sung-hoon, the director for North American trade at the ministry of foreign affairs and trade
One of the most difficult items on the negotiating table will be reducing South Korea's agricultural import barriers, especially for rice, experts said.
Last week, South Korea cleared one hurdle to the start of the talks by reducing quotas aimed at protecting its film industry.
Two-way goods trade between the United States, the world's largest economy, and South Korea, ranked 11th in the world in 2004 by the World Bank, totalled more than $72.5 billion in 2004, with South Korea enjoying a surplus of nearly $20
South Korea has free-trade agreements with Chile, Singapore and the European Free Trade Association made up of Switzerland, Norway, Iceland and Liechtenstein.