President Reagan today urged rejection of a nuclear freeze and pushed for acceptance of his arms control initiatives, saying that the Soviets would agree to them only if the United States and its allies maintain a united commitment to military strength.

In a speech that appeared to be directed as much at his domestic political critics as at the Soviets, the president claimed that he had "launched the most comprehensive program of arms control initiatives ever undertaken" and hinted at new proposals to come.

Reagan also suggested that proponents of a nuclear freeze will bear a heavy responsibility if the superpowers fail to achieve a strategic nuclear arms pact.

"It is vital that we show patience, determination and, above all, national unity," Reagan told an audience of 1,300 persons who listened attentively and applauded the president strongly at the conclusion of his speech.

"If we appear to be divided--if the Soviets suspect that domestic political pressure will undercut our position--they will dig in their heels. And that can only delay an agreement, and may destroy all hope for an agreement," the president said.

Administration officials said that Reagan would soon embrace a proposal by Sens. Henry M. Jackson (D-Wash.) and Sam Nunn (D-Ga.) that would establish a U.S.-Soviet crisis center intended to reduce the risk of accidental nuclear war.

The principal theme of Reagan's expository speech on arms control to the Los Angeles World Affairs Council was that he is a peacemaker whose efforts are made more difficult by well-meaning critics who give the Soviets hope that they can achieve their goals without negotiation.

Democrats praised the president for his efforts to reduce the number of nuclear missiles in Europe, but warned him against attacking proponents of the nuclear freeze.

"Our country's progress toward a solution for this most important of all issues has suffered from domestic divisions that are frequently deepened by attacks on the motivations of those who adopt opposing views," said Rep. Albert Gore Jr. (D-Tenn.), who delivered the Democrats' official response.

Reagan's emphasis on the freeze was directed at the vote that will be taken in the House on this issue after Congress' Easter recess. While administration officials still say they think odds favor the passage of the non-binding freeze resolution, they also say they believe chances of defeating it have been improved by recent demonstrations of the president's flexibility on arms control.

Reagan's speech was worded carefully so as not to challenge the motives of the freeze advocates, who Reagan said are "well-intentioned--concerned about the arms race and the danger of nuclear war."

But he then described the freeze proposal as "dangerous" and said "it would pull the rug out from under our negotiators in Geneva . . . . "

"After all, why should the Soviets negotiate if they have already achieved a freeze in a position of advantage to them?" Reagan said.

In an argument both for his strategy of negotiating from strength and for his embattled defense budget, Reagan insisted that the Soviets best understood the threat of force and would reduce military arsenals "only if they see it as absolutely necessary.

"Only if the Soviets recognize the West's determination to modernize its own military forces will they see an incentive to negotiate a verifiable agreement establishing equal, lower levels," Reagan said. "And, very simply, that is one of the main reasons why we must rebuild our defensive strength."

Emphasizing his oft-stated view that the Soviets are not to be trusted, the president said "there have been increasingly serious grounds for questioning their compliance with the arms control agreements that have already been signed and that we have both pledged to uphold."

Administration officials said the president was referring to reported Soviet deployment of the SS16 missile and the testing of two types of missiles, instead of one, in violation of the SALT II treaty. That treaty was never ratified by the United States but the administration has pledged to comply with its provisions.

Originally, the president's speech here was supposed to provide the platform in which he presented a revised proposal calling for reduction of intermediate-range nuclear weapons in Europe. After Reagan made this proposal earlier in the week, the speech was recast as a comprehensive review of the administration's arms control and national security policies.

Tucked into this summary was a brief statement on nuclear nonproliferation that appeared to signal new administration concern about the spread of nuclear arms.

"Our allies--as important nuclear exporters--also have a very important responsibility to prevent the spread of nuclear arms," Reagan said. "To advance this goal, we should all adopt comprehensive safeguards as a condition for nuclear supply commitments we make in the future."

An administration official said this statement was directed primarily at the French and West Germans, who had urged Reagan to be flexible on arms control. The official added that Reagan intends to raise this issue with European leaders when he meets them just before or during the Williamsburg, Va., economic summit conference late in May.

Reagan offered what appeared to be an olive branch to Congress on the MX missile. The weapon is under study by a presidential commission, which is scheduled to issue a report during the next few days.

The president did not refer to the MX by name, calling it only "the land-based leg of the triad" and saying that it is important to develop a "national bipartisan consensus" on how the MX or an alternative land-based missile should be deployed.

Congress, skeptical of the administration's plan to jam the MX missiles into a single strip of land with the designation "dense pack," handed Reagan a stinging defeat on his MX-deployment plan in December.

"No one gained from this divisiveness; all of us are going to have to take a fresh look at our previous positions," Reagan said. "I pledge to you my participation in such a fresh look and my determination to assist in forging a renewed bipartisan consensus."