President Reagan tried to calm European anxiety about nuclear war today by pledging his commitment to peace and urging that NATO's conventional forces be strengthened as an alternative to nuclear arms.

Before a cheering Bundestag, where he was interrupted by applause 18 times, Reagan declared: "Strong conventional forces can make the danger of conventional or nuclear conflict more remote. Reasonable strength in and of itself is not bad; it is honorable when used to maintain peace or defend deeply held beliefs."

The day before, the touring U.S. president addressed members of the British Parliament, urging a "crusade for freedom" against the Soviet system and praising military preparedness as a means of avoiding war.

Today, Reagan again returned to his favorite foreign-policy theme of peace through preparedness. But in this conflict-conscious capital, only a few hours' drive from the East German border, he emphasized not the menace of communism but the hope that Western unity and military strength would lead to arms reductions and accommodation with the Soviets.

"The nuclear threat is a terrible beast," Reagan said. "Perhaps the banner carried in one of the nuclear demonstrations here in Germany said it best. The sign read, 'I am afraid.' "

"To those who march for peace, my heart is with you. I would be at the head of your parade if I believed marching alone could bring about a more secure world. And to the 2,800 women in Filderstadt who sent a petition for peace to Soviet President Leonid Brezhnev and myself, let me say, I myself would sign your petition if I thought it could bring about harmony . . . . The women of Filderstadt and I share the same goal. The question is how to proceed."

Referring to some U.S. critics who want to bring American troops home or Europeans who see the U.S. presence as more risk than protection, Reagan said such arguments are wrong, but he called on leaders of both countries to help explain the importance of transatlantic partnerships. He cited what he called the sacrifice of U.S. taxpayers in shouldering the burden of common defense and said each member of the alliance should also bear a fair share.

Although Reagan did not yield on any of his basic positions, it was one of the few times that he has expressed any sympathy or understanding for the concerns of the antinuclear movement. These concerns will be voiced Thursday in the streets of Bonn--and again Friday in Berlin--in demonstrations protesting Reagan's military policies.

West Geman Chancellor Helmut Schmidt tried to reassure Reagan about these demonstrations when the two leaders met privately today.

According to government spokesman Klaus Boelling, Schmidt told Reagan that most of those demonstrating were under 30 and were worried about excessive armaments and about peace.

Schmidt said he found it good that many young people today were standing up for peace because 40 years ago many young Germans were applauding Adolf Hitler.

Reagan won his loudest applause when he pledged his support to the Western alliance that was forged in the aftermath of Germany's defeat in World War II.

After saying that the American people support an improved military posture "because they understand how fundamental it is to keeping the peace they so fervently desire," the president pledged his personal backing for the alliance.

"We also are resolved to maintain the presence of well-equipped and well-trained forces in Europe, and our strategic forces will be modernized and remain committed to the alliance" Reagan said. "By these actions, the people of the United States are saying, 'We are with you, Germany. You are not alone.' Our adversaries would be foolishly mistaken should they gamble that Americans would abandon their alliance responsibilities, no matter how severe the test."

A large majority of the Bundestag (the lower house of Parliament) clearly liked what Reagan had to say today, although at least half a dozen members did not rise when he entered, and several others stood without applauding. Two renegade former members of the ruling Social Democratic Party heckled Reagan. One of them, Karl-Heinz Hansen, shouted, "What about El Salvador?"

Reagan ignored the hecklers at first, but when they persisted he finally asked, "Is there an echo in here?" The Bundestag broke into laughter and applause.

At Thursday's NATO summit meeting, Reagan will attempt to score a public relations victory with a proposal, which the administration describes as new, that would reduce the ground forces of both NATO and Warsaw Pact forces in Europe to 700,000 and the combined air and ground forces of each side to 900,000. In fact, a similar proposal was made by the United States in 1975 during the Ford administration and withdrawn in 1979. Reagan made no reference to this.

The present plan is, in the words of one U.S. spokesman, "more streamlined" and takes into account the total forces of the two alliances rather than just U.S. and Soviet troops. The summit meeting also is expected to pledge renewal of allied unity and, in a concession by the Reagan administration, to support detente, redefined as "genuine detente."

Although he stressed U.S. vigilance toward the Soviet Bloc, Reagan said he was "optimistic about our relationship with the Soviet Union if the Western nations remain true to their values and true to each other."

One cloud over today's summit was the condition of Schmidt, 63, who stayed in bed this morning with a fever. Boelling stressed to reporters that this was merely a virus infection and that there was nothing wrong with Schmidt's heart.

Since the chancellor was fitted with a pacemaker last October, there has been heightened concern for his health.

Schmidt was mildly embarrassed during the week by a letter circulated by 59 mostly left-wing members of the chancellor's Social Democratic Party attacking Reagan policies. It described as the greatest worry the "massive arms buildup with mass-destruction weapons such as the ones that your defense minister[Caspar Weinberger] has enforced."

Countering the letter, the Social Democratic floor leader, Herbert Wehner, drew up a motion yesterday that welcomed Reagan to Bonn, pledged West German loyalty to the Western alliance and urged the peace movement to respect the American leader as the freely elected representative of the American people. Only a few Social Democratic deputies abstained when the motion was approved Tuesday.

To underscore their belief that a large majority of West Germans are pro-American, the opposition Christian Democrats staged rallies in Bonn and Munich last Saturday that drew crowds of about 140,000.

The day was not without its pranks and protests.

As Reagan began his speech, half a dozen air raid sirens on the roof of buildings near the Bundestag switched on mysteriously, confounding technicians who took up to an hour trying to turn them all off.

In front of the chancellor's offices, about 80 persons demonstrated against U.S. military policy, carrying posters of Martin Luther King Jr. and Mahatma Gandhi and banners with slogans opposing armaments and war. Among the protesters were Petra Kelly, a leader of the left-wing "Greens" party, Gert Bastian, a retired West German general, Joseph Beuys, an artist, and Rudolf Bahro, an East German critic who was expelled by the East German Communists