President Reagan today denounced Soviet expansionism in Asia, Africa and Central America but called for a "fresh start" in U.S.-Soviet relations through a three-step initiative designed to resolve conflicts in Afghanistan, Angola, Cambodia, Ethiopia and Nicaragua.

The new proposal was the centerpiece of Reagan's speech to the U.N. General Assembly on a busy day of diplomacy. The president also held a "mini-summit" with leaders of five industrialized democracies and met for 30 minutes with Soviet Foreign Minister Eduard Shevardnadze.

In his 30-minute address on the 40th anniversary of the founding of the United Nations, the president said that "true peace" and progress at next month's Geneva summit with Soviet leader Mikhail Gorbachev was possible only through candid discussion of U.S.-Soviet differences and a "realistic view of the world."

This view, as expressed by Reagan, holds the Soviets responsible for continuing conflicts in Third World trouble spots, which the president said "are the consequence of an ideology imposed from without, dividing nations and creating regimes that are . . . at war with their own people."

"And in each case," the president added, "Marxism-Leninism's war with the people becomes war with their neighbors," Reagan added.

In a speech his advisers said was designed to recapture the public relations initiative for the United States and emphasize Soviet vulnerability, the president stressed regional conflicts and subordinated discussion of arms control except for promoting his Strategic Defense Initiative (SDI).

The president proposed "a regional peace process" in which combatants in the five specified countries would begin direct negotiations with a view to "achieving an end to violence, the withdrawal of foreign troops and national reconciliation."

Once these negotiations were under way, the United States and the Soviet Union would hold separate discussions to encourage the bargaining of the warring parties.

If these first two steps proved successful, Reagan said, "the United States would respond generously" by "welcoming each country back into the world economy so its citizens can share in the dynamic growth that other developing countries . . . enjoy."

Reagan did not specify what this "welcoming" would mean, but a senior administration official said it would include U.S. economic aid to the five nations once the disputes were resolved and all foreign troops withdrawn.

Except for some perfunctory clapping by Soviet Ambassador Anatoliy Dobrynin, the Soviet delegation headed by Shevardnadze sat silently through Reagan's speech and did not applaud when he had finished.

Tass, the official Soviet news agency, in its first reaction to Reagan's U.N. speech, did not applaud either. According to Reuter, Tass said the speech offered nothing new and reaffirmed U. S. Policies of "banditry."

Tass said that instead of standing up for the rights of sovereign nations, Reagan tried "to whitewash the undeclared wars unleased for the purpose of suppressing peoples' strugles for freedom and progress," Reuter said.

Shevardnadze, speaking to the General Assembly two hours after Reagan's address, agreed with the need to end "small wars" but blamed the United States for many of the world's regional problems.

Specifically, he referred to Vietnam, "where neither people nor the environment have been able even now to recover from the consequences of the barbarous aggression which has maimed the entire country with napalm and chemical agents."

Shevardnadze also said that in Afghanistan and Nicaragua "the bullets of hired assassins . . . are killing thousands of people."

Western leaders, including West German Chancellor Helmut Kohl and Israeli Prime Minister Shimon Peres, expressed cautious approval of Reagan's speech. Privately, some Western and Third World diplomats said they were surprised at the president's harsh tone toward Moscow, in view of the conciliatory tone he took on regional issues in a U.N. speech last year.

Today, however, as one West European diplomat put it, "Reagan offered only to talk about those disputes involving the Russians and their allies, and only after the pro-Soviet governments embark on talks with rebel movements supported by the West."

In his address, Reagan staunchly defended his proposal for a space-based strategic missile defense system, frequently called "Star Wars." He cited former Soviet premier Alexei Kosygin's advocacy of a missile defense system in 1967, when the proposal was opposed by the Johnson administration.

Reagan presented Star Wars in its most visionary form, as a space shield that would protect the civilian population from destruction, a goal that even its advocates acknowledge is decades away.

"Until that day, the United States seeks to escape the prison of mutual terror by research and testing that could, in time, enable us to neutralize the threat of these ballistic missiles and, ultimately, render them obsolete," Reagan said.

Addressing a series of rhetorical questions to the Soviets on a subject likely to be a vital issue at the Geneva summit Nov. 19-20, Reagan said: "How is Moscow threatened if the capitals of other nations are protected? We do not ask that the Soviet leaders -- whose country has suffered so much from war -- leave their people defenseless against foreign attack. Why then do they insist that we remain undefended?"

Administration officials said that Reagan, who agreed Wednesday with British Prime Minister Margaret Thatcher that the Soviets have scored public relations points in criticizing Star Wars, wanted to use the opportunity of the U.N. speech to make the strongest possible case for his missile defense proposal.

Reagan said that missile defense had the potential to lift "the sword of Damocles that has hung over our planet for too many decades" and prevent space from being used as an avenue for mass destruction.

"If we are destined by history to compete, militarily, to keep the peace, then let us compete in systems that defend our societies rather than weapons which can destroy us both and much of God's creation along with us," Reagan said.

Secretary of State George P. Shultz provided the Soviets with an advance copy of Reagan's speech and Shevardnadze returned the courtesy before he spoke. Despite the sharp differences betwen the U.S. and Soviet positions, Shultz said that the Soviets had agreed to discuss regional issues at the summit without ruling anything off the agenda.

Reagan's speech today differed from his three previous addresses to the United Nations in both tone and purpose. His first speech, on June 17, 1982, made many of the same points expressed today but in blunt and sometimes bristling language that reflected the absence of any U.S.-Soviet negotiations at the time.

In the 1982 speech Reagan accused the Soviets of using "chemical weapons against the freedom fighters of Afghanistan," and of supplying toxins for use in Cambodia and Laos. Today, Reagan made the same points but in moderate language, saying that he wanted to discuss "violations" of a 1972 international treaty banning biological and toxin weapons when he meets Gorbachev in Geneva.

Reagan's speeches at the United Nations in 1983 and 1984 discarded much of the anti-Soviet rhetoric and dealt largely in generalities rather than specific proposals.

Officials said that Reagan's speech today, intended to be substantive in purpose but unprovocative in tone, reflected the reality of the forthcoming summit in which the United States and the Soviets are competing for world public opinion.

"When Mr. Gorbachev and I meet in Geneva next month, I look to a fresh start in the relationship of our two nations," Reagan said. "We can and should meet in the spirit that we can deal with our differences peacefully. That is what we expect."