President Reagan led a chorus of civilian and military leaders who paid tribute to Vietnam veterans and welcomed them back into the national fold in ceremonies yesterday at the Vietnam Veterans Memorial and Arlington National Cemetery.

"You are forever in the ranks of that special number of Americans in every generation that the nation records as true patriots," the president said to cheers from an estimated 150,000 veterans and others who gathered at the memorial on the Mall.

It was a day filled with echoes of Vietnam, from the presence of the white-clad corps of Gold Star Mothers whose sons had died there, to the surprise appearance at Arlington National Cemetery of the former commander of U.S. ground forces in Vietnam, retired general William C. Westmoreland, who was greeted with cheers from his former troops.

Reagan took the occasion to sound a theme of reconciliation between the nation and veterans who served in a war that "threatened to tear our society apart.

"We can forgive ourselves . . . for those things that may have been wrong in the conduct of the war ," he said. "There has been rethinking on all sides and this is good. It is time we moved on in unity and with resolve."

The president said that in the years since the war, Vietnam veterans have not received the due gratitude of the nation.

" . . . Some of your countrymen were unable to distinguish between our native distaste for war and the stainless patriotism of those who suffered the scars," he said. "There has been a rethinking there too. Now we can say to you and say as a nation, thank you for your courage, thank you for being patient with your countrymen."

Completing his speech, Reagan signed documents formally turning the Vietnam Veterans Memorial over to the Department of the Interior. The "Three Fightingmen" statue by Washington sculptor Frederick Hart, unveiled Friday, was formally dedicated at the ceremony.

The president's remarks were delivered somberly and several times Reagan appeared choked up. The large crowd, cordoned away from the platform and from the memorial itself, stretched eastward from the area along the Reflecting Pool and Constitution Avenue. On several occasions during his address, the audience shouted approval and applauded. When Reagan offered his support for continued efforts to locate the estimated 2,500 servicemen still missing in action, shouts of "bring them back" were raised in the crowd.

Reagan's appearance at the ceremony was not announced until Friday, although organizers of the event said they had made it known to White House staff that his presence would be welcomed. The president's decision to give a speech, just five days after his landslide reelection victory, was viewed by officials of the Vietnam Veterans Memorial Fund as an effort to bind up old wounds and reconcile veterans of different generations.

Jan Scruggs, the Vietnam veteran who conceived the memorial in 1979, told the crowd that he found some irony in the event, however.

"It seems ironic . . . . Instead of the Vietnam vets building a memorial and giving it to the government," he said, "the government should have built a memorial and given it to the vets."

Also appearing with the president and his wife, Nancy, who joined a group of dignitaries on the speakers' platform, was Interior Secretary William Clark.

Clark invoked the words of Abraham Lincoln by saying, as Lincoln did at Gettysburg, that the memorial grounds were already consecrecated "far beyond our poor power to add or detract."

Earlier in the day, Secretary of Defense Caspar Weinberger laid a presidential wreath at the Tomb of the Unknowns at Arlington and afterward told a boisterous crowd of 5,000 in the cemetery amphitheater, "We must never again send Americans into battle unless we plan to win" -- a comment that won him a standing ovation.

The wreath-laying ceremony at 11 a.m. began with a 19-gun salute that thundered under gray skies, while wispy clouds swept past the colonnaded monument to America's unidenitifed war dead.

In his speech, Weinberger recalled the sacrifices of all veterans and singled out the contribution of American's 1.2 million women who have served in the armed forces.

The defense secretary asserted there is a changed public attitude toward the military more than a decade after Americans withdrew from Vietnam.

"There is one burden that these young active duty Americans do not bear today . . . . They don't have to face the boos and the picket signs that so many of you had to face," he said.

Westmoreland's surprise appearance at the morning ceremony won him a rousing reception. Introduced to the crowd, he responded to shouts of "Speech! Speech!" with a few impromptu remarks.

"You did the job your country asked you to," he said, addressing the Vietnam veterans. "You did it magnificently and nobody could have done it any better."

Reagan's address climaxed a weekend of activities organized to coincide with the final step toward the establishment of the Vietnam Veterans Memorial. Two years after the dedication of the granite wall that bears the names of the 58,022 war dead, veterans organizers staged reunions, prayer sessions, vigils and a concert featuring Frankie Valli, a singer whose music was at the height of its popularity in the Vietnam Era.

At the conclusion of yesterday's events, the entire memorial -- including the statue, the granite wall, the flagpole and two acres of parkland -- was transferred to the federal government to be cared for by the Park Service.

The transfer signaled the end of a five-year campaign to build a memorial to veterans of the Vietnam conflict.

The campaign was marked by the same pluralism of opinion that characterized debate over the war itself. Led by Scruggs, an Army veteran who was wounded in battle, the fight to erect the monument provoked a series of controversies. The design of the memorial, conceived by Yale architectural student Maya Ling Lin, triggered a volley of caustic criticism by those who believed the memorial, in which the names of those killed or missing in action are inscribed on black granite, was too abstract and not heroic. One critic called it a "black gash of shame and sorrow."

Opponents of "the wall" moved to block its execution, but a compromise was worked out in which a realistic statue by Hart was added to the memorial. The "Three Fightingmen" statue, which depicts white, black and Hispanic servicemen, itself was attacked by some who thought it was an unnecessary addition to the memorial. But it was hailed by Scruggs yesterday as "beautiful."

"We gave our country back its pride. We gave our country back its unity -- both of which were so badly wounded during that war," he said.

Vietnam veterans arriving in Washington for this year's events at the memorial were more upbeat and less angry than two years ago, according to organizers of "Salute II," as the weekend events were called. Several speakers yesterday attributed the change to the progress of a "healing process" begun when "the wall" was dedicated.

"I believe that in the decade since Vietnam, the healing has begun," the president said. "And I hope that before my days as commander-in-chief are over, the process of healing will be complete."

When the speeches ended, the crowd moved down into the low ground where the V-shaped memorial rests in a grass-covered berm. Veterans, friends and families poured down the walkway and stood, six and seven deep, facing the wall as darkness fell and faint beams from recessed lights illuminated the names on it.