With a brisk exchange of pens and handshakes, President Reagan and Soviet leader Mikhail Gorbachev signed a historic treaty at the White House yesterday eliminating an entire class of nuclear-armed missiles and vowed to make progress toward another treaty that would slash the strategic nuclear arsenals of the rival superpowers.

In an East Room ceremony that was solemn and celebratory, the two leaders sat side by side at a table once used by President Abraham Lincoln and put their signatures to the accord to do away with medium- and shorter-range nuclear-tipped missiles.

"We can only hope that this history-making agreement will not be an end in itself, but the beginning of a working relationship that will enable us to tackle the other . . . urgent issues before us," Reagan said, before signing the first major treaty of his presidency.

He said these issues include "strategic offensive nuclear weapons, the balance of conventional forces in Europe, the destructive and tragic regional conflicts that beset so many parts of our globe, and respect for the human and natural rights that God has granted to all men."

Gorbachev, responding, said "we can be proud of planting this sapling which may one day grow into a mighty tree of peace."

"May Dec. 8, 1987, become a date that will be inscribed in the history books -- a date that will mark the watershed separating the era of a mounting risk of nuclear war from the era of a demilitarization of human life," Gorbachev said.

Scores of seated officials, dignitaries, and guests of the two leaders looked on as they carefully inscribed their names eight times in two large copies of the treaties, one bound in slate-blue leather for the United States, the other in burgundy-red leather for the Soviet Union. Nancy Reagan and Raisa Gorbachev sat beside each other as their husbands signed.

The accord will require the two countries to eliminate missiles, deployed and undeployed, of 300- to 3,000-mile range. The United States has 859; the Soviets 1,752, according to U.S. officials.

The treaty faces a contentious ratification process in the U.S. Senate, but is assured unanimous approval by the Supreme Soviet, Gorbachev's rubber-stamp national parliament.

After signing the Intermediate-Range Nuclear Forces (INF) Treaty, the two leaders walked together to the State Dining Room and made separate televised statements for a worldwide audience.

Then, they held the second of the summit's substantive meetings, which focused on strategic arms and other arms reduction issues. White House spokesman Marlin Fitzwater said the signing ceremony had given "a boost to the hope for progress" in these areas but added that a "sense of political realism" soon entered the picture.

In all, Reagan and Gorbachev spent almost three hours in official talks yesterday, beginning with a one-on-one morning discussion in the Oval Office and an afternoon plenary session in the Cabinet Room attended by senior advisers. Fitzwater later characterized the summit talks as "a day of positive, productive discussions."

Following a familiar summit format, task forces on arms reduction issues and on other issues of the broad U.S.-Soviet agenda were established, and these groups of senior officials met yesterday afternoon and in some cases last night. No results of these meetings were made public.

By the end of the day, Fitzwater said, "both sides felt that a foundation had been laid for substantive progress on specific issues." He gave no details.

A White House official who briefed reporters later on condition he not be identified said Gorbachev offered "no surprises or new proposals." The official said Reagan had opened the day's talks by raising questions of Jewish emigration and divided spouses, and that Gorbachev countered by saying the Soviets were seeking to improve in these areas, and also raised questions about U.S. human rights practices.

The extraordinary events of this busy and historic day gave Americans the best look they have ever had at the 56-year-old Russian who is shaking up the Soviet system and who has favorably impressed much of the noncommunist world.

The Gorbachev who was on public display on the White House South Lawn, in the East Room, the State Dining Room and in a late afternoon meeting with private Americans at the Soviet Embassy was a man of many faces. He was at times intensely serious and businesslike, speaking of war and peace and the destiny of peoples; at times gently joshing with Reagan and playing to audiences and cameras with humor and burlesque; at times seeming almost to overflow with words and gestures in an extemporaneous, almost imploring appeal to American intellectuals to bury the attitudes and images of the past and turn a new eye to a changing future.

In his White House remarks and at the Soviet Embassy, Gorbachev made the case for his restructuring in the Soviet Union, which he calls "perestroika." Internationally, he declared, "Something very serious is afoot, something very profound . . . . {There is} an awareness that we cannot go on as we are, that we cannot leave our relations the way they are."

Americans also received a glimpse of a president who appeared to have recovered the confidence that had deserted him during a year marked by congressional and criminal investigations into the Iran-contra affair, the collapse of the stock market and the cancer surgery of Nancy Reagan.

The 76-year-old president, appearing good-humored and relaxed, seemed to enjoy his exchanges with Gorbachev and to relish their appearance on the world stage together. The two leaders referred frequently to their ideological differences without rancor and stood solemnly together at the morning arrival ceremony as bands played first the Soviet and then the American national anthems.

But the treaty-signing ceremony also provided a glimpse of some of Reagan's potential political problems as the Senate considers ratifying the INF pact. Sen. Jesse Helms (N.C.), ranking Republican on the Foreign Relations Committee, was invited to the White House but declined.

In their public exchanges throughout the first day of the three-day summit, Gorbachev and Reagan displayed a comfortable familiarity with each other even while differing on substantive issues.

In the signing ceremony Reagan quoted what he called "an old Russian maxim" that he has frequently used in his speeches, "doveryai, no proveryai -- trust, but verify."

Gorbachev interrupted with a smile, saying, "You repeat that at every meeting."

Reagan, turning to the Soviet leader, said, "I like it."

The exchange was greeted with laughter and applause by the U.S. and Soviet officials gathered in the East Room.

Reagan and Gorbachev vied with each other in their worldwide televised speeches to express a sense of optimism about further agreements and an improvement in U.S.-Soviet relations while also warning that many differences remain between the superpowers.

"The treaty just signed in Washington is a major watershed in international development," Gorbachev said. "Its significance and implications go far beyond what has actually been agreed upon." In words similar to those used by the president, Gorbachev called the INF treaty "only a beginning" and said it had been achieved through "lengthy and intense arguments" that overcame "long-held emotions and ingrained stereotypes."

The day began with a 10 a.m. arrival ceremony on the South Lawn of the White House where Gorbachev was given a trumpet fanfare and the honor of a 21-gun salute.

He and his wife Raisa arrived at the South Entrance in a black, bulletproof Soviet Zil limousine bedecked with Soviet and American flags. They were welcomed by the President and Mrs. Reagan and a crowd of several hundred U.S. and Soviet officials waving paper flags of both nations.

The panoply of the day's events was in sharp contrast with the early stages of the Reagan presidency, which began with a burst of anti-Soviet rhetoric and the largest peacetime U.S. military buildup in history.

Yesterday, a Marine Band in the foyer of the White House alternated playing of American and Soviet marches before Reagan and Gorbachev strode together down a red-carpeted hallway into the East Room as an announcer intoned, "Ladies and gentlemen, the president of the United States and the general secretary of the Communist Party of the Soviet Union."

They were greeted by a standing ovation from an audience that included the U.S. Joint Chiefs of Staffs and Marshal Sergei Akhromeyev, chief of staff of the Soviet armed forces.

Reagan began his remarks by recalling the long road the two leaders had traveled to the completion of the INF treaty.

"This ceremony and the treaty we are signing today are both excellent examples of the rewards of patience," the president said. "It was over six years ago, Nov. 18, 1981, that I first proposed what would come to be called the zero option. It was a simple proposal -- one might say, disarmingly simple. Unlike treaties in the past . . . it didn't simply talk of controlling an arms race. For the first time in history, the language of arms control was replaced by arms reduction."

Reagan acknowledged that reaction to the proposal had been "mixed" and said that "to some, the zero option was impossibly visionary and unrealistic; to others, merely a propaganda ploy. Well, with patience, determination and commitment, we've made this impossible vision a reality."

The Soviet leader, addressing his remarks to "Mr. President, ladies and gentlemen, comrades," said the treaty had a "universal significance for mankind, both from the standpoint of world politics, and from the standpoint of humanism."

Speaking animatedly in Russian that was translated into English, Gorbachev said, "The treaty whose text is on this table offers a big chance at last to get on the road leading away from the threat of catastrophe. It is our duty to take full advantage of that chance, and move together toward a nuclear-free world which holds out for our children and grandchildren, and for their children and grandchildren, the promise of a fulfilling and happy life without fear and without a senseless waste of resources on weapons of destruction."

For all the celebratory spirit, Reagan and Gorbachev were not above competing in direct and scrappy fashion for favor with the other's constituency.

Reagan, speaking to the Soviet people in his televised statement after the treaty signing, said, "The true America is not supermarkets filled with meats, milk and goods of all description; it is not highways filled with cars," managing to suggest the bounty of the United States even while denying that this is its spirit.

Gorbachev, in turn, spoke to the American people of an idyllic world "in which American and Soviet spacecraft would come together for docking and joint voyages, not for Star Wars," managing to take a poke at the popular name for Reagan's cherished Strategic Defense Initiative (SDI).

A last-minute hitch that threatened to mar the proceedings was settled yesterday morning when the Soviets provided a clear photograph, transmitted by facsimile from Moscow overnight, of an SS20 missile, the principal Soviet weapon to be eliminated by the accord.

The United States, lacking its own clear photo of the mobile SS20, had demanded one from the Soviets for inclusion in an INF document, but rejected the Soviet's first response because it showed the weapon hidden inside a cannister.

Another unexpected development was the 11th hour U.S. decision not to publish a "memorandum of understanding" negotiated as part of the treaty, which gives the locations of U.S. and Soviet missiles to be scrapped and of procedures for verifying their destruction and inspecting the sites from which they are to be removed.

A senior State Department official said the material was withheld at the request of the Defense Department, where some officials are concerned the details in the document could be used by terrorists.

The working group on arms control, headed by U.S. arms adviser Paul H. Nitze and Soviet Marshal Akhromeyev, met for several hours yesterday afternoon. Officials said they expect more meetings of the working group today.

Akhromeyev, highly regarded by U.S. officials as a tough, but authoritative negotiator, will receive unprecedented access to the U.S. military, going to the Pentagon to see Secretary of Defense Frank C. Carlucci today, Joint Chiefs of Staff Chairman Adm. William J. Crowe Jr. Thursday, and the National Military Command Center, heart of Pentagon operations.

Another U.S.-Soviet working group headed by Assistant Secretary of State Rozanne L. Ridgway and Deputy Foreign Minister Alexandr Bessmertnykh is considering regional disputes, human rights and bilateral issues. This group began meeting in the late afternoon and was reported to be holding discussions into last evening.

Americans at the White House state dinner said the day's talks had gone well. "They have a way of talking to each other," one participant in the leaders' expanded exchanges said. Another official said the atmosphere was so positive "the biggest problem is to keep enthusiasm down" and move slowly.

One veteran said such high-level meetings cannot be judged the first day, despite positive signs. "The real test will come in the middle of the night on Wednesday," said another, speaking of the strategic arms cuts.

The meeting of Reagan and Gorbachev this morning is to center on such regional conflicts as Afghanistan, the Persian Gulf, southern Africa and Central America, Fitzwater announced.

Staff writers David Hoffman and Molly Moore and staff researcher Michelle Hall contributed to this report.