National Park Service workers took hammers and crowbars to protest signs in Lafayette Park yesterday, tearing down the much-criticized, billboard-type structures that have sprung up in recent years across the street from the White House and setting off an emotional confrontation between antinuclear activists and a conservative group that came to cheer the demolition.

"Go, go, go, go!" about a half-dozen members of Young Americans for Freedom chanted as the crudely constructed plywood signs came crashing down.

Several times during the hour-long dismantling of nearly a dozen so-called "Peace Park" protest displays, the YAF and other conservative representatives, calling themselves the Lafayette Liberation League, engaged in heckling and a sing-off of sorts with those who stood protectively by their signs.

"Hey, don't you guys know the '60s are over?" Jay Young, YAF's national director, shouted as protesters began singing "Give Peace a Chance." Soon, however, the young conservatives had raised their own voices in song and were trying to drown out the peace anthem with a chorus of "God Bless America."

But despite this musical clash and several more heated exchanges between supporters and opponents of the protest displays, enforcement of the Park Service's new regulations restricting the number and size of protest signs in the park went off smoothly.

One demonstrator, Robert Dorrough, 26, was arrested when he refused to move away from one of the peace signs that he said he was "defending." He was charged with violating the new sign size limitations, which became effective yesterday, a federal misdemeanor punishable by a fine of up to $500 and six months in jail if he is convicted.

Work crews began dismantling the signs and their elaborate support bracings at 10:45 a.m. They also carted off several boxes of food, reading material, clothes and trash stored behind the protest displays, leaving only three small signs that were judged to conform with the new regulations.

Under the new rules, free-standing signs in the park must be no larger than four feet in either dimension. No protester may have more than two such signs in the park at any time, and a protester must remain within three feet of the sign or it will be considered abandoned property and removed. Hand-carried signs are exempted from the size restrictions.

The Park Service proposed the regulations last August, complaining that the large, permanent protest displays created a "dump-like" atmosphere in the historic park and blocked the view of the White House. Officials said the problem had worsened since 1983, when permanent protest displays were banned from the sidewalk in front of the White House, resulting in increased protest activity in the park.

But while many applauded the new rules, others said that the park was the best place to catch the attention of the president and the public and that restricting the size and number of protest signs would curb free speech.

The Washington office of the American Civil Liberties Union has not challenged the regulations but is watching to see how they are enforced and what effect they have on protests in the park, according to legal director Arthur B. Spitzer.

As their signs were torn down yesterday, two longtime antinuclear demonstrators watched stoically from the sidewalk but said they had no intention of abandoning the protest they began five years ago.

"I will return with regulation signs," said William Thomas, 39, who has filed suit in federal court alleging that these and previously adopted protest restrictions are part of a government conspiracy against free expression.

A hearing on his suit was postponed yesterday by U.S. District Judge Louis F. Oberdorfer.

Concepcion Picciotto, 42, who maintains a 24-hour vigil in the park against nuclear weapons, got into a brief shouting match with YAF members and spent the rest of the morning giving interviews and passing out antinuclear leaflets.

"So much work, so much effort," said Picciotto, dressed in her trademark wig, head scarf and green parka, as the signs -- most of which belonged to her -- were carted off to three waiting trucks.

The YAF members, who brandished tape measures and moved from sign to sign checking their dimensions, were unmoved. "This contributes to the homeless by one," a group member said.

The dismantling of the protest displays drew about 200 onlookers, including the usual assortment of office workers, winos and tourists who share the park in nice weather. They gawked, giggled or took sides as sign proponents and opponents faced off in angry debates. Visitors from an English prep school and a Girl Scout troop snapped more pictures of the scene than they did of the White House.

At 11:50 a.m., when the final illegal sign was knocked down, most of the crowd applauded. "I defend to the hilt their right to have some kind of statement," said John Grigsby, 22, another member of YAF. "But this is such an extreme perversion of free speech."

Bob Condry, 36, who stopped to watch the gathering, got drawn into the dispute when he overheard one YAF member call the protesters "communists."

"I'm in support of removing the signs, but I think you can take stands on free speech without being labeled a communist," he said, his neck veins bulging as he argued with the YAF group. "We're all Americans . . . . I'm just going to follow them around and counteract everything they say."