MOSCOW, NOV. 7 -- The Soviet Union celebrated its 70th anniversary today with a Red Square parade that mixed the traditional display of military might with slogans for reform.

As a light snow drifted across the historic square, Soviet leader Mikhail Gorbachev climbed to the top of the Lenin Mausoleum for his third review of the Revolution Day parade since he became general secretary of the Communist Party.

The themes of the Gorbachev era predominated today as marchers in the parade carried banners heralding "democracy" and "openness." On the square, on either side of a giant portrait of Vladimir Lenin, hung two posters, one listing the slogans of the 1917 revolution, the other proclaiming the watchwords of perestroika -- Gorbachev's "restructuring" program.

Following the elaborate protocol of the most sanctified of Soviet holidays, Defense Minister Dmitri Yazov gave the day's only speech. Yazov accused "reactionary imperialist circles" of continuing to strive for military superiority and pledged to keep Soviet defenses at the necessary level.

Yazov said the Soviet Union was doing everything to "prevent the realization of designs of aggressive imperialist circles," and cited the coming summit meeting in Washington between Gorbachev and President Reagan next month.

Besides signing a treaty eliminating medium- and shorter-range nuclear weapons, the two sides will consider "the question of a would-be treaty" cutting back strategic arsenals by 50 percent and agreeing on mutual compliance with the Antibalistic Missile treaty "for a period to be agreed upon," Yazov said.

Yazov, an alternate member of the Politburo, became defense minister following the ouster of his predecessor after a West German teenager landed a small plane on Red Square on May 28. Today, Yazov acknowledged that the armed forces are going through further changes, saying the military is experiencing its own "restructuring."

"Like the country on the whole, it {is} directed at enhancing in all ways vigilance and combat readiness," Yazov said, according to the news agency Tass.

The parade concluded a week-long celebration of the 1917 revolution. It brought communist and leftist leaders to Moscow from around the world and gave Gorbachev a chance to outline his evolving concept of Soviet-style socialism.

History and the experience of the last 70 years were themes of many of the public discussions and press conferences held during the week. Today, history was put on parade along with the customary tanks and rockets, floats and banners.

The first regiments to march past the mausoleum wore uniforms of the revolutionary era and the Russian civil war. There were the leather suits of the Red Guards and the great winter coats of the Red Army, as well as cavalrymen on horses and tachankas -- machine guns on carts pulled by four horses.

As in other years, the parade also featured troops in World War II uniforms and the "banner of victory," flown by Soviet soldiers over the Reichstag in Berlin in 1945.

The only new piece of military equipment presented today was the Soviets' latest armored personnel carrier. Otherwise, the hardware, including a Scud missile, was the same as in other years.

The decorations for the parade this year differed slightly from past years. The giant picture of Lenin in red was a photograph of the Bolshevik leader rather than the usual idealized portrait.

The number of slogans read out over loudspeakers was reduced, reflecting a new emphasis on simpler rhetoric.

After the military display was over and exhaust from the tanks had cleared, the square filled with marching "demonstrators" -- workers from Moscow plants and factories who are bused in to parade.

This year, the floats again emphasized the elements of reform, each proclaiming success in making economic gains.

Also present on the mausoleum were Cuban President Fidel Castro and four East European Communist leaders -- Wojciech Jaruzelski of Poland, Todor Zhivkov of Bulgaria, Erich Honecker of East Germany and Nicolae Ceausescu of Romania.