The Soviet Union, playing host to the 12th World Youth Festival, celebrated its opening today with an extravagant pageantry usually reserved for organized sports.

The kickoff for the week-long festival was presided over by the ruling Politburo, including Soviet leader Mikhail Gorbachev, who took a day off from his vacation to attend. He told the crowd to work for a "future of mankind without war, violence and oppression."

This is the first youth festival to be held here since 1957, when thousands of young Soviets mingled with young foreigners, opening the first crack to the outside world after the death of dictator Joseph Stalin.

During today's four-hour ceremony, as thousands in the packed 100,000-seat Lenin Stadium flashed color cards on cue, about 150 national delegations paraded behind their flags, some dancing in native costume, some chanting slogans or carrying banners.

The staging of the event and Gorbachev's presence underscored the political investment Moscow has made in the festival, which is organized around the slogan, "For antiimperialist solidarity, peace and friendship."

Western critics have charged that the festival's official sponsor, the World Federation of Democratic Youth, touts the strict Soviet line, restricts opposing views and uses the festival to slam the United States and its allies. The Soviet press says these charges are the product of a "vicious antifestival campaign" spawned by western propaganda agencies.

About 20,000 participants were expected at this year's festival, the first since Cuba hosted the 11th festival in 1978. Most come from communist or leftist parties and, judging from the faces, many are pushing the age limit of 39 used by the festival to define youth.

The festival is taking place under strict controls, as Moscow police and security forces leave nothing to chance. The press has even hinted that good weather would be assured by seeding rain clouds before they could reach the scene. Fears that foreigners will transmit diseases to young Muscovites have led to unusual cautionary articles in youth newspapers about such ailments as AIDS.

Nonresidents have been barred from the city, and motorists have been advised -- sometimes strenuously -- to use public transportation. Gorky Park, Moscow University, the festival clubs and other key sites are off limits to all but those accredited to the festival.

The festival's political character is clear from the program, which has scheduled workshops, seminars and rallies on "the struggle of the people, youth and students of Central America and the Caribbean against interventionist policy of U.S. imperialism, fascist and dictatorial regimes," and an "anti-imperialist tribunal," with hearings on the arms race and other subjects usually tagged to the United States in Soviet propaganda.

Politics have already cropped up in the nightly meetings of the preparatory committee. Moderate leftist groups, particularly from Europe, succeeded in providing for "free tribunes," where people can talk on any subject. These meetings have not been prominently advertised, however, and are restricted to one place, to three days and to specific hours.

There have also been conflicts over the use of the word "Zionist." A compromise would refer only to present-day expansionist policies, rather than a blanket reference to historical Zionism. That apparently was inadequate for the Libyans, who have pulled out of the festival.

Also not coming are the Albanians and the Chinese, who over the years have objected to the heavily pro-Soviet bias voiced at previous festivals.

The American delegation represents the Young Communist League, civil rights groups, trade unions and representatives of several members of Congress. Angela Davis of the U.S. Communist Party is an honored guest. Bernice King, daughter of civil rights leader Martin Luther King Jr., is a delegate.

The American club, housed in a neighborhood palace of culture, has an exhibit on the ground floor with posters for protagonists and causes spanning decades of American leftist politics: Karen Silkwood, John Lennon, Daniel Berrigan, antiwar marches, civil rights marches.

Some participants in the festival take care to note that their presence does not mean endorsement of all its themes. David Woollcombe, coauthor of the rock musical "Peace Child," said he came with 17 young American actors because it was his only chance to bring his show here.

"The last thing I wanted was to get involved with the festival," he said, "because it is something where they can use me more than I can use them. But it was the only way to get them involved."

Moscow's restaurants have been painted and the streets hung with multicolored banners and the festival emblem, a dove inside a rosette. Tourism virtually shut down to make room for the visiting delegations; some airlines canceled flights because no hotel rooms were available.

More than 700,000 young people have left the city for vacations timed to coincide with the festival, according to one press report.

The head of the local Young Communist committee was particularly concerned about young "problem teen-agers," and called for a "significant" increase in law enforcement -- particularly in hotels and youth cafes.

To help clear the streets of traffic -- already reduced by the ban on out-of-town vehicles -- motorists are stopped for the slightest infraction and have their licenses suspended for the duration of the fesival.

During the week, there will be concerts and cultural programs, pavilions along the Moscow River in Gorky Park, where people can meet and talk.

Twenty-eight years ago, 34,000 came to the sixth world festival here. It made an indelible impression on a host population that had been shut off from the outside world.

"It was the biggest holiday of my life," said an artist, now 47, warming to the memories. "You can't imagine what Moscow was like. Whenever three people gathered, there was a concert. Someone would have a guitar. Someone would sing."

"There had been practically no contact before -- no travel, very few tourists," said a journalist, now 67. "For most people, the festival was the first time they met a foreigner."

Few expect this year's festival to have the same impact. Muscovites are now used to foreigners. In summer, the Intourist buses pack the key tourist spots, and hotels are full. An estimated 30,000 foreigners are in residence here.

But while Muscovites are more blase than they were in 1957, they could not ignore the preparations for the festival. On the one hand, residents seemed pleased that the city was closed to the daily droves of people from the provinces who normally come to shop. And, as during the Olympics, people took note of the improved quality and greater quantity of goods and foods in the stores -- tasty sausages, Belgian shoes and fruits. Said a teacher, "It shows that they can do it when they want to."

On the other hand, the gradual clamping down of the city began to get on people's nerves. Much of downtown was closed for parking and would-be drivers were crowding public transport. Restaurants were closed for preparations, and along the main streets, endless columns of trucks and buses circled in dry runs for the real event.