President Reagan said tonight the Soviet Union has "begun to make a serious effort" to negotiate reductions in nuclear weapons and called on Soviet leader Mikhail Gorbachev to agree soon on a planning meeting for another superpower summit.

Reagan struck an openly conciliatory tone toward Moscow in a commencement address at Glassboro High School, saying recent developments on arms control suggest "this can be a moment of opportunity" in U.S.-Soviet relations.

Recalling the June 1967 summit here between President Lyndon B. Johnson and Soviet Premier Alexei Kosygin, Reagan declared, "I have come here today to say the Glassboro summit was not enough, that indeed the Geneva summit was not enough, that talk alone, in short, is not enough. I have come here to invite Mr. Gorbachev to join me in taking action, action in the name of peace."

In his first comment on the Soviet arms control proposal presented recently in Geneva, Reagan said, "We cannot accept these particular proposals without some change, but it appears that the Soviets have begun to make a serious effort.

"If both sides genuinely want progress, then this could represent a turning point in the effort to make ours a safer and more peaceful world," he said. "We believe that possibly an atmosphere does exist that will allow for serious discussion."

Reagan's remarks came as the administration is under strong criticism in Congress and among the allies over the decision to abandon arms limits in the unratified SALT II treaty. Tonight's speech appeared to be an effort to defuse the controversy while also urging the Soviets to move forward on planning the next summit.

While repeating his criticisms of Soviet expansionism and totalitarianism, Reagan tonight muted many of his past complaints about Moscow. For example, he did not mention alleged Soviet violations of the SALT II agreement, which were the basis for his decision to abandon its limits.

Reagan attempted tonight to look beyond the SALT II decision. "Let us leave behind efforts to seek only limits to the increase of nuclear arms and seek instead actual arms reductions," he said. He also appealed for support of his proposed missile defense system, the Strategic Defense Initiative, a "shield that could protect us from nuclear missiles just as a roof protects a family from rain."

Sources said earlier today that White House officials had a strong internal disagreement over how Reagan should characterize the new Soviet proposal in his speech, with some suggesting a harder line. But Reagan intervened personally, they said, with the conciliatory language in tonight's address.

Until recently, Reagan said, Soviet moves on arms issues had been "disappointing," but recently "there have been fresh developments," including Soviet proposals on reducing conventional forces in Europe, on nuclear power plant safety after the Chernobyl accident and at the arms talks in Geneva. The new Soviet proposal there linked U.S. adherence to the 1972 Anti-Ballistic Missile Treaty to reductions in strategic weapons.

Soviet diplomats have said that a positive signal from Reagan on the arms proposal could clear the way for agreement on a summit planning meeting. Tonight Reagan urged such a planning meeting soon between Secretary of State George P. Shultz and Soviet Foreign Minister Eduard Shevardnadze.

"The location is unimportant," Reagan said, referring to the planning meeting. "What matters is that such a meeting take place in mutual earnestness so we can make progress at the next summit."

Noting that Johnson and Kosygin met for longer than expected here, Reagan said that historians have concluded the summit "was not, in fact, one of the most momentous -- no major breakthroughs were made or agreements reached.

"Nevertheless, the two men met," he said. "They were frank. They worked to understand each other and to make themselves understood. In this alone, I would submit, they taught us a great deal."

Reagan has been unable in the 5 1/2 years of his presidency to reach any major arms-control agreement with the Soviets. But tonight he said it was his hope that others who look back on his presidency "will say we worked to break the patterns of history that all too often resulted in war, that we reached for accord, that we reached for peace."

"There can . . . be no more important task before us than that of reducing nuclear weapons," Reagan said. "I am committed -- utterly committed -- to pursuing every opportunity to discuss and explore ways to achieve real and verifiable arms reductions."