Sister Lucia, 44, stood with other nuns from Sisters of the Korean Martyrs Catholic convent in the cold evening, holding a candle and shouting toward the U.S. Embassy, ringed by riot police.

"Yankee Go Home!" she chanted, arm raised.

On the lapel of her coat, over her religious habit, was a pin reading "[Expletive] USA." She shrugged, unapologetic. "Someone was handing it out."

Such sentiment has surged here, and it is mixing with politics to create a volatile strain of anti-Americanism in South Korea, where 37,000 U.S. troops remain facing North Korea.

South Korean anger over a gruesome traffic accident in which two young girls were killed -- and two U.S. soldiers were acquitted by a military court -- has fed into the frenzy of a presidential campaign. Both candidates in the Dec. 19 election to replace outgoing President Kim Dae Jung have ratcheted up their rhetoric, criticizing the rules under which U.S. troops are stationed here and demanding a personal apology from President Bush for the accident.

Both Kim and the media are calling for restraint, but thousands of protesters took to the streets in Seoul on Saturday night, blowing whistles, carrying candles and shouting denunciations of the American presence.

Protests aimed at the U.S. military, which remained here after the 1950-53 Korean War, have waxed and waned in South Korea. There were violent anti-American demonstrations in the 1980s. But some longtime observers are worried at the breadth of anger over what one diplomat called "the most dramatic events involving the U.S. troops here in 50 years."

"Anti-American sentiments have spread into almost all strata of Korean society," said Kim Seung Hwan, writing in the Korea Times. "It's growing at a startling rate."

The demonstrators include students and leftists long opposed to U.S. troops here. About 50 protesters stormed an American base near Seoul Nov. 26, cutting the fence, and others threw gasoline bombs into a second base. There were no injuries.

But protests are now being joined by nuns such as Sister Lucia, Buddhist monks, pop stars, teachers, movie idols and citizens who normally would not be on the picket lines. The demonstrations reflect a creeping anger at what the protesters describe as a growing American arrogance worldwide. "This anti-Americanism is hurting our national interest," President Kim warned Friday. The protests prompted the cancellation of a congressional delegation to be led by Rep. Henry J. Hyde (R-Ill.), which was due in Seoul on Saturday.

The mood is being reflected in the presidential election campaign, a tumultuous affair in which the candidate leading in the polls has -- for the first time in South Korea -- uncertain enthusiasm about the existing partnership with the United States.

Roh Moo Hyun, 56, a rough-talking, self-taught lawyer who has represented labor activists and student dissidents, has made speeches urging that U.S. troops leave South Korea. He has since retreated from that stand, saying it was youthful passion. But he remains critical of what he describes as the subservient relationship of his country to the United States.

He hit a popular chord. His polls spiked earlier this year when he criticized South Korean politicians for their traditional pilgrimage to the United States to get "a political blessing" from Washington. He has never been to America.

Roh has said he would essentially continue Kim's "sunshine policy" toward North Korea, which has put the government here at cross-purposes with Bush and his refusal to talk with North Korea. "I frankly have worries about the hard-line policies of Bush," Roh told a news conference last week.

His opponent, opposition party chief Lee Hoi Chang, 67, is a staunch conservative much more in tune with Bush's view of North Korea. But Lee, too, has bent to the winds of public opinion in an effort to capture the pivotal middle ground of the electorate.

He has said he would be willing to meet the North Korean leader, Kim Jong Il. He has joined Roh in calling for a revision of the Status of Forces Agreement, under which two U.S. servicemen involved in the traffic accident were tried in a U.S. military court. And he, like Roh, has said the statement of apology read on behalf of Bush by the U.S. ambassador here Nov. 27 -- in which Bush said he was "touched by this tragedy" -- is insufficient. He insists Bush should say the words himself.

"Our people should no longer be humiliated," Lee said while campaigning Friday in a southwestern province. "America still does not comprehend. Their leader, President Bush, should directly apologize to our people."

Lee's drift makes some in his conservative camp uncomfortable, acknowledged one strategist in Lee's Grand National Party. But Lee is struggling to rise above a consistent support rate of about 35 percent. Both camps say their internal polls show Roh ahead. Under campaign rules, the South Korean media cannot publish poll results so close to the election.

The challenge for both candidates is to judge how closely to embrace the current public anger. Lee needs to keep -- and Roh wants to win -- voters who remember the U.S. sacrifice during the Korean War and feel the demonstrations show ingratitude for American troops who remain on the front line, deterring North Korea.

But there is little doubt that loyalty to America -- long the rule in South Korea -- has been soured. The tragic accident last June 13 has cast a dark shadow over views of the United States.

On that day, a U.S. Army infantry division moved huge armored mine-clearing vehicles from their base toward training grounds outside Seoul. The cause of the accident is still uncertain. But the soldiers took a road too narrow for the big vehicles, rounded a blind curve too fast and communications with the driver were faulty.

Two schoolgirls, Shim Mi Son and Shin Hyo Sun, 14, walking on the pedestrian shoulder of the road, were crushed under the wheels of one of the machines.

Because the accident happened while they were on duty, the driver, Sgt. Mark Walker, and the commander of the vehicle, Sgt. Fernando Nino, were charged in a U.S. military court, instead of a Korean civilian court. After separate trials, on Nov. 20 and 23, the military court found them not guilty on charges of negligent homicide. They were quickly transferred out of the country.

The public reaction was fury. Critics called the process a sham, "a trial of an accomplice by accomplices." Protests erupted, demanding justice for the "murders" of the girls.

"If they had been found guilty, there wouldn't have been protests. . . . people saw the decision as very unfair," said Lee Chung Hee, a political science professor at Hankuk University of Foreign Studies.

"Emotions ran wild. Koreans can get spun up," said a Western diplomat. "This incident has damaged the image of the U.S. military in Korea with a wide swath of the population. It's a scar that will last."

Cultural differences also played a role. In South Korea, and in many Asian nations, such an incident would prompt the quick resignation of the military commander or a top political officer to take responsibility. But no one was publicly punished. The U.S. Army even refused to say if administrative action was taken, citing the privacy of soldiers.

Public emotions also were inflamed by photos of what was left of the girls' bodies. The grim pictures, which would not be printed by a newspaper, quickly spread on the Internet.

"It was the image," said Hahm Chai Bong, a political science professor at Yonsei University. "It brought out the middle class, and the middle-schoolers." But he noted that the image was only a spark of deeper frictions in the society, largely the same divisions that will decide the presidential election.

Younger South Koreans, in their twenties and thirties , and some in their forties -- what one called "the McDonald's generation" -- do not feel that the threat from North Korea still justifies the presence of U.S. troops in their country.

"Koreans have been living with a dual personality -- a strong sense of nationalism and an American presence here. It's got to grate on you," said Hahm.

Special correspondent Joohee Cho contributed to this report.

Roh Moo Hyun, the leading presidential candidate, has been critical of U.S.-South Korean relations.Opposition party presidential candidate Lee Hoi Chang, upper left, attends a memorial service Saturday in Seoul for two schoolgirls killed by a U.S. military vehicle.A protester demanding an apology from President Bush for the military accident that killed two Korean girls is arrested Saturday in front of the White House.