MOSCOW -- For the old Arbat, one of this city's most famous 19th-century neighborhoods, glasnost summer had come to mean a daily festival of spontaneous street activity with artists churning out portraits on demand and strolling musicians drawing crowds with ballads that gently mocked the problems of everyday Soviet life.

But perhaps the most extraordinary phenomenon of all was the small band of Hare Krishna believers, who every night since early July would gather in front of the Vakhtangava Theater, singing Buddhist mantras and swaying in unison. When they finished their singing, the group members were invariably approached by curious onlookers who wanted to learn who they were and what they believed in. The conversations between atheists and believers, soldiers in uniform and hippies with long hair often lasted late into the night.

Then Monday, the tolerance that allowed such a flourishing street life reached its limit. Fifteen of the Hare Krishna followers, who had been celebrating the birthday of their spiritual founder, were taken to a local militia station and accused of violating Article 193 of the administrative code forbidding religious observances anywhere but in a church or temple.

The problem for the Hare Krishna group is that Soviet authorities have never acknowledged them as an official religious sect, even though the group has been applying for this status for the last six years.

"If we are an official group, then we are allowed to worship only in a temple," noted Alexander Dragilyov, a 21-year-old member of the group. "But if we are not an official religious group, then we should be allowed to sing on the street."

But they are not allowed, according to the message delivered by the militia Monday. The group was told that if they went back onto the Arbat to sing and dance again, they would be arrested and charged under the criminal code, Dragilyov said.

Dragilyov said the trouble actually began Sunday night when a group of youths grabbed one of the Hare Krishna banners and ran off. He said several onlookers went up to uniformed militiamen standing nearby and asked them to do something about this, and they were told, "It is not our business."

On Monday night, a group of youths Dragilyov believes were the same ones from the night before approached the chanting sect members outside the Vakhtangava Theatre and turned their tape decks up to full volume. At the same time, a 15-member military orchestra positioned near the theater began to play. The Hare Krishna mantras were drowned out.

A short time later, a number of uniformed militiamen came up and took the group's members by the arms, leading them off to a nearby militia station. There they were held for three hours and made to sign documents charging them with administrative violations.

Similar actions have been taken recently against other Hare Krishna groups in the country -- in Kiev, capital of the Ukraine, and in Lithuania, Dragilyov said. According to the group's estimates, there are 2,000 members of the sect in the Soviet Union, of whom 25 are now in prison or in labor camp.

The group's problems are symptomatic of the tenuous nature of the ongoing Soviet experiment with open debate. In the days when they were peacefully singing their mantras, undisturbed by authorities, the reaction of passers-by varied, but most shared the view of one young man who said, "As long as they don't bother anyone, then what is the harm?"

Sometimes arguments would break out, as a Communist Party veteran would query the young Krishna adherents about the sincerity of their values and, in Russian fashion, launch debates about Asian versus European cultures and the essence of man's spiritual needs. The gatherings had also begun to attract other religious groups, including members of fundamentalist Christian sects -- all this in a society officially wedded to atheism.

Some passers-by began to wonder how long this would go on. "Something seems to be missing," said one young Russian recently, looking over his shoulder for the usually ubiquitous militiamen. Yet for most of the summer, the Arbat was relatively free of uniforms.

According to Dragilyov, the militia's sharp warning Monday is not something the group is eager to trifle with. "We won't sing on the Arbat," he said yesterday, "but we will go there and walk."

The same delicate balance between law and order and new forms of expression has been tested elsewhere in Moscow this summer. One western diplomat witnessed a group of young, inebriated sailors clogging nearby Kalinin Prospekt on Navy Day, singing and chanting vaguely risque' refrains ("Hail to documentary films -- nyet. Hail to artistic films -- nyet. Hail to pornographic films -- da"). The group was broken up without incident as it neared the entrance to the Kremlin.

In another part of the city, militiamen have twice come to call on Sergei Grigoryants, editor of the unofficial journal Glasnost, which has now issued a second edition with articles on a range of sensitive topics, from the KGB to the destruction of archives documenting cases of political repression in the 1930s and 1940s.

Both times, the police left, once when western television cameras showed up and the second time when Grigoryants said they were coming.

In the meantime, Grigoryants -- now a celebrity who is met by visiting U.S. senators interested in following the course of glasnost -- received a notice from his district government committee, asking him to register the journal.

"What it means, I don't understand," he said over the telephone yesterday. "I tried calling the district committee all day but couldn't get through.