SEOUL, June 11 (Xinhua) -- Elements of at least 80,000 protesters against South Korea's recent agreement to resume U.S. beef imports raised makeshift styrofoam stairs early Wednesday to climb a bulky container barricade set up to prevent their march on the presidential complex.

    More than a dozen protesters waved flags, including the national flag, and unfurled a banner criticizing President Lee Myung-bak, after reaching the top of the five-meter barricade at the landmark Sejonro Intersection, about a kilometer away from the presidential palace, Yonhap news agency reported.

    The protesters also criticized other major plans by the new government, including a cross-country canal project and a set of pro-business economic reforms.

    The rare scene at the sand-filled containers came early Wednesday morning after scuffles erupted among protesters with no serious injuries reported as they debated for hours whether they should override safety concerns to stage their opposition to the April 18 deal between Seoul and Washington.

    Protesters argue that the deal fails to protect South Koreans from mad cow disease because it allows beef from cows 30 months old or above while parts like bones can be imported.

    Mad cow disease, also known as bovine spongiform scephalopathy, mostly has been found in cattle over the age, while specified risk materials, including intestines, tongues and tails, pose greater risk of transmitting it.

    The barricade marked the first time that South Korean police have used containers in the capital, as previous protests involved demonstrators using ropes to pull police buses and clear their way to the presidential palace.

    The scene was reminiscent of a 2005 anti-globalization protest when a similar barricade was set up against farmers and activists in South Korea's second largest city Busan during a meeting of Asian Pacific leaders there.

    The rally that began on Tuesday in Seoul, which police estimated 80,000 joined, was the largest ever since candlelight vigils began early last month against the April 18 agreement.

    Amid the daily protests calling for a better deal, Lee phoned his U.S. counterpart George W. Bush on Saturday and asked him to only allow the export of beef made from cattle younger than 30 months.

    A top presidential advisor and a group of ruling party lawmakers also left for Washington on Monday to follow up on the phone conversation, while the Cabinet has offered to resign over the political crisis aggravated by nose-diving public approval ratings for Lee.

    The rally in Seoul came on the anniversary of a historic 1987 protest that led the then-military regime to grant free presidential elections and prompted a series of democratic reforms.

    The National Police Agency initiated the highest alert that allows about 40,000 riot police officers to be mobilized nationwide when authorities believe public security is severely threatened.

    Scientists believe mad cow disease leads to the spongy-like degeneration of the brain, medically referred to as variant Creutzfeldt-Jakob Disease, when a human contracts the disease by eating infected food.

    About 200 people have died around the world from the disease, but according to the U.S. government, no human infection has been reported from eating American beef.

    South Korea has declared it will indefinitely delay putting theApril deal in effect until Washington or U.S. exporters agree to refrain from exporting beef from cows aged 30 months or older. Butit has not addressed the controversy over parts to be imported.

    The U.S. side has yet to respond formally to South Korea's request.