Speaking to a group of businessmen at his office, Mr. Lee gave his first comment on the massive rally against his four-month-old government that brought at least 100,000 people into the streets of Seoul on Tuesday and prompted his entire cabinet to offer to resign.

The beef protests have dealt a sharp blow to Mr. Lee, who was elected in December championing a new “pragmatic” approach to ties with Washington.

He made rebuilding South Korea’s political and economic alliance with the United States his top priority, while taking a much harder line on North Korea than his predecessor, Roh Moo-hyun.

Bush administration officials have expressed hopes that Mr. Lee’s firm stance on North Korea’s nuclear program, which reversed South Korea’s previous policy to embrace its neighbor, could persuade the North to end its nuclear program. North Korea promised to dismantle its nuclear weapons facilities under an international accord that has yet to achieve lasting results.

Both Mr. Lee and President Bush also hoped that Mr. Lee’s decision in April to end the five-year ban on American beef would help win support in Congress for a free-trade agreement struck between the governments last year, thus improving relations while helping to revive the sluggish South Korean economy.

But some South Korean analysts say Mr. Lee may now come under pressure to take a less accommodating line with Washington.

Mr. Lee was himself a former student activist imprisoned by the country’s then military regime. During the current protests, many student protesters called Mr. Lee “authoritarian” and in his comments Wednesday the president appeared to have understood the irony.

“As a former participant in a pro-democracy student movement myself, I had many thoughts watching yesterday’s demonstration," Mr. Lee was quoted as saying by his office. "My government intends to have a new beginning with a new resolution."

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Seoul reverberated with antigovernment slogans until well past midnight. While people marched by candlelight, loudspeakers blared the songs South Koreans used to sing during their struggle against the military dictators of the 1970s and 1980s.

The protests Tuesday took place on the 21st anniversary of the huge pro-democracy demonstrations that helped end authoritarian rule. Overhead, balloons carried banners that said "Judgment day for Lee Myung-bak" and "Renegotiate the beef deal." One widely distributed leaflet said, "Mad cow drives our people mad!"

The agriculture minister, Chung Won-chun, visited the protest site to offer an apology in a speech, but protesters quickly surrounded him, chanting "Traitor!" and he was forced to leave.

Mr. Lee urged the police and protesters to avoid clashes. He promised to be "humble before the people’s voices" and called for national unity to overcome an economic crisis spawned by stagnant growth and surging prices for oil and other raw materials.