The walkout came a day after American beef went on sale in one store, where the owner says the meat was "selling like hotcakes."
U.S. beef is usually considerably cheaper than domestic meat, with prices as low as one-fourth that of Korean beef depending on cut and quality.
Prime Minister Han Seung-soo was one of the first customers, buying 26 pounds from the A-Meat store in Seoul — the only place currently selling U.S. beef.
"I had American beef together with my grandson and other family members yesterday," Han's office quoted him as saying during a meeting with a local religious leader. "It was nice and tasted good."
Store owner Park Chang-gyu said about 880 pounds of meat was sold Tuesday.
"U.S. beef is selling like hotcakes," Park said. He added however that anti-U.S beef protesters held a demonstration in front of the store Wednesday, causing sales to be halted for about three hours.
The walkout by auto workers was part of a one-day strike by the Korean Confederation of Trade Unions to protest U.S. beef imports and economic reform proposals by Lee, who took office in February.
About 55,000 workers at South Korea's largest carmaker, Hyundai Motor Co. and its affiliate Kia Motors Corp., stopped work for two hours, union officials said. An additional 23,000 Hyundai and Kia workers on the night shift planned to do the same later.
The strike was expected to cost the automakers about $40 million in lost production of 2,900 vehicles, a Hyundai spokesman, Jake Jang, said.
The KCTU said about 130,000 of its 600,000 members were expected to join the strike, including workers at textile and chemical factories.
About 5,000 people gathered Wednesday evening under rainy skies in central Seoul, police said, with no clashes immediately reported. Some KCTU members also took part in the candlelight rally in front of Seoul's City Hall, according to police.
"This is not a political strike, but a strike that is aimed at protecting our right to health," KCTU leader Lee Suk-haeng told reporters. "We want to live long and healthily."
Lee said his group plans to launch a nationwide consumer campaign to boycott American beef.
The president's office denounced the walkout as an attempt to hurt the nation's economy and vowed to deal sternly with strikes.
"Now it is time for everyone to return to work and pull together all our energies and wisdom to surmount economic difficulties," the office said in a statement.
U.S. beef imports to South Korea have been largely banned since 2003, when the first case of mad cow disease was discovered in the United States. President Lee agreed to lift the import ban in April just before a summit with President Bush.
But the move provoked a backlash due to health concerns and a perception that South Korea had backed down too easily to American pressure.
As the protests peaked in June at 80,000 people, the Cabinet offered to resign and Lee reshuffled top aides. Seoul negotiated an amendment to the import deal last month to limit shipments to beef from cattle younger than 30 months, believed less susceptible to mad cow disease.
Since the changes to the deal, the daily rallies have dwindled in size although they have become more violent.
Rally organizers said the smaller protests do not mean the public is now satisfied with the safety of U.S. beef and that they would continue demonstrating until the agreement was renegotiated to add more safeguards.
"The candles can be put out only when President Lee Myung-bak apologizes sincerely and renegotiates" the beef deal with Washington, said Nam In-soon, a leader of the People's Association For Measures Against Mad Cow Disease, a coalition of civic groups.
In Washington, the White House announced that President Bush would visit South Korea on Aug. 5-6 before heading to the Beijing Olympics. Bush had originally been expected to go to Seoul next week when he visits Japan for the G-8 summit, but the trip did not materialize amid the protests.