The President and the Soviet leader, beginning three days of talks aimed at even broader reductions, pledged to build on the accord by striving toward what Mr. Gorbachev called  »the more important goal, » reducing long-range nuclear weapons.

In their White House conversations, the leaders were said to have reviewed their previous proposals aimed at furthering those negotiations, and they established an arms-control working group of ranking officials to hold parallel sessions.

‘Mine is Mikhail’

An immediate mood of warmth was established as the two leaders agreed this morning to call each other by their first names, a White House official said. He quoted the President as telling Mr. Gorbachev,  »My first name is Ron. »

Mr. Gorbachev answered,  »Mine is Mikhail. »

 »When we’re working in private session, » Mr. Reagan reportedly said,  »we can call each other that. »

The new treaty, which provides for the dismantling of all Soviet and American medium- and shorter-range missiles, establishes the most extensive system of weapons inspection ever negotiated by the two countries, including placing technicians at sensitive sites on each other’s territory.

The Mood for Talking

The signing, the fruition of years of negotiation, set the mood for two and a half hours of talks between the leaders. The talks were  »very serious, substantive discussions, » Secretary of State George P. Shultz said tonight before a formal dinner in the White House.

The visit to Washington by Mr. Gorbachev was the first by a Soviet leader since Leonid I. Brezhnev was here 14 years ago, and it took on immediate drama as Mr. Reagan, who entered office with deep suspicions of the Soviet Union, welcomed Mr. Gorbachev on the South Lawn of the White House.

 »I have often felt that our people should have been better friends long ago, » he told his guest as they stood facing the Washington Monument across an array of full-dress military honor guards. Mr. Gorbachev received a 21-gun salute usually reserved for chiefs of state.

Mr. Gorbachev plunged energetically into a round of talks and public appearances. He met twice with the President, attended a formal dinner at the White House and met with a group of American public figures and intellectuals at the Soviet Embassy.

‘Lively’ Rights Discussion

The morning discussion, in which Mr. Reagan raised human rights issues, was  »a very lively session, » according to Marlin Fitzwater, the White House spokesman. He described Mr. Gorbachev as  »animated » and Mr. Reagan as  »forceful. » Another official said there was  »no give » by Mr. Gorbachev on Jewish emigration or other rights issues.

Mr. Reagan and Mr. Gorbachev both exuded warmth, with Mr. Reagan quoting Russian proverbs, Mr. Gorbachev quoting Ralph Waldo Emerson and both men exchanging frequent smiles in what seemed to be a spirit of satisfaction and expectation.

Seated side by side at a massive wooden table used by Lincoln’s Cabinet, they signed English- and Russian-language versions of the treaty at a ceremony in the East Room of the White House, attended by about 250 invited guests, including the Joint Chiefs of Staff, the Cabinet, Congressional leaders, ranking Soviet officials and others.

 »We can only hope that this history-making agreement will not be an end in itself, » President Reagan said,  »but the beginning of a working relationship that will enable us to tackle the other issues, urgent issues, before us: 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. » The Road from Catastrophe

Mr. Gorbachev echoed some of those sentiments.  »For everyone, and above all for our two great powers, » he said,  »the treaty whose text is on this table offers a big chance, at last, to get onto 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. »

If approved by the Senate, the accord would require the dismantling within three years of all 1,752 Soviet and 859 American missiles with ranges of 300 to 3,400 miles and their nuclear warheads. It also provides for stationing inspection teams at sensitive sites on each other’s soil, with the right to make a certain number of short-notice inspections elsewhere each year for 13 years.

Although much debate is expected in the Senate, Republican and Democratic leaders there say they believe the treaty will be approved, perhaps with amendments, reservations or understandings attached. Senator Bob Dole, Republican of Kansas and the minority leader, who is running for President, has remained uncommitted.

Jesse Helms of North Carolina, the ranking Republican on the Senate Foreign Relations Committee, is strongly opposed to the accord. He declined an invitation to attend the signing ceremony, a White House official said.

Perplexed by Opposition

Mr. Gorbachev, meeting with a group of American intellectuals and public figures, expressed bewilderment at the opposition to the treaty.  »I cannot comprehend those who have taken up the cudgels against the newly found elements of mutual understanding and cooperation that have appeared in preparation for the signing of the treaty, » he said.

Mr. Reagan, who had some sharp exchanges with conservatives on the subject last week, laced his statements today with references to the need for realism and the avoidance of illusions, an apparent effort to allay the concerns of some conservatives that he is growing soft on the Russians.

 »I have often felt that our people should have been better friends long ago, » Mr. Reagan declared during the arrival ceremony on the South Lawn.  »But let us have the courage to recognize that there are weighty differences between our Governments and systems – differences that will not go away by wishful thinking or expressions of good will, no matter how sincerely delivered. »

At various times, both leaders referred to the Soviet-American alliance against Germany during World War II. And Mr. Gorbachev, at the arrival ceremony, also summoned the call of history to enhance the aura of this summit meeting.

 »History has charged the Governments of our countries, and the two of us, Mr. President, with a solemn duty to justify the hopes of Americans and Soviet people, and of people the world over, to undo the logic of the arms race by working together in good faith. »

A choice was offered, Mr. Gorbachev declared:  »Fears and prejudice inherited from the cold war and leading to confrontation, or common sense, which calls for action to ensure the survival of civilization. »

At the signing ceremony, Mr. Reagan emphasized the extensive verification procedures that would enable both sides to monitor compliance with the treaty.  »We have listened to the wisdom in an old Russian maxim, » Mr. Reagan said.  »Though my pronunciation may give you difficulty, the maxim is, ‘Doveryai no proveryai,’ ‘trust but verify.’  »

Mr. Gorbachev interrupted, laughing.  »You repeat that at every meeting, » he said.  »I like it, » Mr. Reagan replied. Mr. Gorbachev, who arrived in Washington on Monday afternoon, began his public day with a full-dress welcoming ceremony on the South Lawn. There were military honor guards, cannon salutes, trumpet fanfares and the playing of the two countries’ national anthems.

Then Mr. Gorbachev and Mr. Reagan met privately in the Oval Office, each accompanied only by two note-takers and an interpreter. They were joined later by senior aides. Mr. Fitzwater, the White House spokesman, said they discussed human rights and arms control and reviewed the agenda, setting up two working groups.

One group, on arms control, is headed by Paul H. Nitze, special adviser to the President, and Marshal Sergei F. Akhrameyev, chief of the Soviet General Staff. Another group, to deal with regional conflicts, human rights and bilateral issues, is headed by Rozanne L. Ridgway, Assistant Secretary for European and Canadian Affairs, and a Deputy Soviet Foreign Minister, Aleksandr A. Bessmertnykh.

The two leaders held talks again this afternoon.

Although Mr. Reagan has said he would press the Soviet leader on human rights, the President made only brief public remarks on the issue.

 »On the table will be not only arms reduction, but also human rights issues about which the American people and their government are deeply committed, » Mr. Reagan said at the arrival ceremony.  »These are fundamental issues of political morality that touch on the most basic of human concerns. »

In a broadcast address after the treaty signing, the President declared:  »Let us remember that genuine international confidence and security are inconceivable without open societies with freedom of information, freedom of conscience, the right to publish and the right to travel. »

The missile treaty eliminates only a small percentage of weapons that are not very important to either country, and it leaves untouched the vast long-range strategic nuclear forces of each side. But the accord is widely seen as an important symbolic and psychological step. Some conservatives oppose it in the belief that Soviet compliance cannot be verified and that it leaves Western Europe exposed to a superior Soviet conventional force.

Mr. Gorbachev said today that he hoped to work toward agreements on reducing conventional forces in Europe, as well as a treaty eliminating chemical weapons.

 »We have some real, legitimate concerns » about the arms treaty, Senator Dole said.  »The bottom line for most Republicans is that we want to support the President; we will do our Constitutional duty, and we see no reason why the role must conflict. »

Mr. Dole announced the formation of a Republican task force to coordinate  »expeditious consideration » of the treaty.

At the East Room signing ceremony, with Nancy Reagan and Raisa Gorbachev looking on from the front row, the two leaders laboriously put their signatures to the parchment-quality paper on which the treaty is printed.

Then, with an impish smile, Mr. Gorbachev asked Mr. Reagan if he might like to exchange pens, so each could have the other’s as a souvenir. They did, shook hands and walked from the room down a magenta-carpeted hallway.