When Muscovites stroll - and poke fun at the system

''Hail blue jeans!'' ''Hail rock-and-roll!'' ''Hail Billie Holiday!'' ''Hail Redbone!'' ''Hail . . . hail . . . hail,'' comes the hearty, giggly reply from dozens in the huge crowd surging from Red Square.

''Hail to the Soviet militia!'' cries another voice, drawing more ''hails,'' and more giggles.

''They're pretending it's an official demonstration,'' a young girl explains, unnecessarily, to a friend who is already laughing aloud.

The scene is Moscow, on a perfect spring night. The date: May 1, International Workers' Day. The occasion: May Day's traditional mass gulyaniye - or stroll - through a central Moscow closed to its habitual honking, smoking, screeching traffic.

There is nothing like it on the Soviet calendar: Alone among officially sanctioned events, the May Day gulyaniye admits a measure of spontaneity.

Also, a measure of vodka.

The gulyaniye belongs not to the vetted party faithful - as does the formal Red Square march May Day morning before assembled Soviet leaders - but to more ordinary folk. There are parents with small children. And old women: in Russian, the babushki, or grandmas.

Most of all, there are the teen-agers - and most of these, in Moscow fashion, stroll arm-in-arm, boy with boy, girl with girl.

The rite offers a peek at one way many ordinary people here - not officials, and by no means dissidents - relate to their nation's rigid political system.

They knock its pomp and slogans. Sometimes they even gently test its limits. But it is all in good fun, all with the innate sense there is no percentage, no point, in serious political probing.

The event begins with tens of thousands of Muscovites walking to Red Square down the commercial avenue called Gorky Street. Soldiers channel the crowd into a neat stream at the approach to the square. Unlike a contingent at the daytime march, the troops don't appear armed. In any case, the strollers take no real notice of them.

At 10 p.m. sharp, when most of the teen-agers seem to have a few hours' vodka under their belts, a brief fireworks display begins.

''Hail! . . . Glory!'' shout Russians young and old. Some teen-agers giggle in counterpoint.

The mood of the crowd is polite, relaxed, good-natured.

And it stays that way even as the strollers wheel back down Gorky Street, and the younger people in the crowd begin chanting a litany of distinctly un-Soviet slogans:

''Hail Gorky Street!'' shouts an unsteady youngster with curly blond hair that makes him look something like a Slavic version of the folk-rock singer, Art Garfunkel.

''Hail blue jeans!'' he adds. He sports a pair of genuine American Wranglers, a prized possession here.

''Hail . . . hail . . . hail!'' reply dozens of nearby strollers.

''Hail rock-and-roll. The best music!'' a taller, less hoarse comrade ventures in passable English, drawing an echo of responsive ''hails,'' this time mostly from the teen-agers.

''Peace, yes!'' says another voice, again in English.

''Hail Billie Holiday!'' exclaims Garfunkelsky in Russian.

''Hail Redbone!''

Then, perhaps caught momentarily in the rhythm of his own slogans, he cites a pop group with what is locally a more ambiguous and sensitive title:

''Hail America!'' he ventures, too hoarse by now, or maybe still too sober, to have managed much more than a whisper.

''Calm down,'' a friend counsels quietly. The blond abandons the slogan.

Later, another person, also apparently an acquaintance, tries subtly to steer Garfunkelsky away from the crowd.

At first he hesitates. Then - after a seemingly amiable, if not too coherent debate - he agrees, leaving to await another May Day.