President Lee Myung-bak took office in February on a wave of popularity propelled by his vow to boost South Korea's economy with skills honed in business. He soon led his conservative party to win control of the legislature in national elections.

But a misreading of the public mood over imports of U.S. beef has sent Lee's approval ratings into a nosedive, leaving him caught between a promise to a key ally and the anger of his own people.

After weeks of protests, the government announced Tuesday — Lee's 100th day in office — that it was backing away from an agreement to resume imports of American beef, which was banned after the United States reported its first case of mad cow disease in late 2003.

Agriculture Minister Chung Woon-chun said the government was acting to "humbly accept the people's will," following large street protests over the weekend.

Though Lee's margin of victory in December's presidential election was the biggest ever in South Korea, the beef issue has evaporated his popularity. A poll published in the national newspaper JoongAng Ilbo on Tuesday said less than 20 percent of Koreans support him.

"Lee cannot properly run the country with such a low approval rate," said Im Hyug-baeg, a professor of politics at Korea University.

The president's troubles began April 18, just hours ahead of a meeting with President Bush at Camp David, Md. Seeking to remove a major thorn in their relations, the two countries announced that South Korea would resume importing American beef.

But the deal struck a nerve in South Korea, where it was seen as a brazen attempt to curry favor with Washington and win support from members of Congress for a free trade agreement between the two nations.

People felt Lee acted rashly and ignored their concerns about health.

"The president neglected the process of collecting people's opinions in the course of the beef negotiations," college student Ahn Kwang-soo said at a rally Saturday that drew some 40,000 people. "He should have been more careful."

Critics also complain that rather than acting as the president of a nation, Lee has behaved like the corporate CEO he once was, treating South Koreans more like employees than citizens.

Lee, 66, tried to make amends last month, going on national television to apologize for his handling of the beef deal. The response from the public was an escalation in protests.

The decision by Lee's government to postpone implementing the agreement is likely to raise hackles among members of Congress from cattle states who say a free trade agreement with South Korea has little chance of passing unless U.S. beef returns to South Korean markets.

"I think that any delay in implementing agreements certainly doesn't help win new friends for the FTA," U.S. Ambassador Alexander Vershbow said Tuesday.

Lee's sudden fall from grace is all the more stunning given his legendary status in South Korea as a can-do businessman whose reputation for getting things done won him the nickname "The Bulldozer."

Talent and ambition took him from humble roots to become CEO of a major construction company at age 35, unusual in seniority-dominated South Korea.

He later went into politics, serving in the National Assembly and eventually as mayor of Seoul. He is credited with making the city of over 10 million people more livable by emphasizing environmental improvements such as planting trees and restoring an ancient stream.

Escalating anger over beef has also brought out frustrations on other issues, some beyond Lee's control.

For example, his campaign vow to boost annual economic growth to 7 percent has run into soaring global prices for oil and other commodities.

"I don't think he is reviving the economy and consumer prices are rising sharply," said office worker Choi Won, who was at an anti-government rally under rainy skies Tuesday night.

Holding candles under their umbrellas, some 8,000 protesters chanted: "Lee Myung-bak, step down."

The conservative newspaper Chosun Ilbo, South Korea's largest, said Lee must act quickly to salvage the rest of his five-year term.

"He has no choice but to make a bold decision to change the state of affairs by humbly reflecting on his mistakes and coldly analyzing the factors that pushed him and his administration to this point," the paper said in an editorial.