“The government will actively pursue measures for farmers and smaller business owners to help improve their competitiveness,” said Choe Guem-nak, the president’s spokesman. “We will also make sure that the free trade agreement will rejuvenate the economy and above all, create jobs for young people.”

Mr. Lee’s rapport with President Barack Obama bore fruit last month when the U.S. Congress passed the free-trade deal by a wide margin despite underlying concerns about the effect it might have on U.S. manufacturing, notably the car industry. The deal was approved by Congress while Mr. Lee was visiting Washington.

“This was an inevitable action we had to take, because we could not make one step of progress in our talks with the opposition, which thought only about its partisan interest,” Kim Ki-hyun, a governing party spokesman, said of the hurried vote. “But we apologize to the people for failing to have a negotiated approval of the deal.”

“We apologize to the people for failing to stop this coup,” said Sohn Hak-kyu, head of the main opposition Democratic Party. “From now on, we will fight to have the deal nullified.”

Farmers’ groups across the country issued statements accusing the government of “giving up our economic sovereignty.”

The Korea Chamber of Commerce and Industry said in a statement welcoming the bill’s approval that the deal would “pave an economic highway” to the United States and make South Korea a trading hub for Asia, Europe and North America.

South Korea, a major exporter of industrial goods like automobiles and consumer electronics, has aggressively sought free-trade deals and has several in effect, with countries including Chile, India, the 10-member Association of Southeast Asian Nations and the European Union. South Korea hopes the deal with the United States will go into effect Jan. 1, said Cho Byung-jae, spokesman for the Foreign Ministry.