Seven spinning columns of fire light the late night sky as the Back Alley Street Circus performs the grand finale of its Saturday evening juggling show. "Thought you had the good seats, didn't you," shouts Ned Van Alstyne at the closest onlookers. A bit worried, one boy jumps up from his spot on the Brattle Square traffic island and hurries across the street toward Bailey's. Van Alstyne and his partner finish their startling display, extinguish the last flaming baton, and begin circulating their matching black derbies among the applauding crowd of 100 people.

"Please, folks, be courteous. Let those shy people with the five-dollar bills get through," bellows a smiling Van Alstyne.

The two-man Back Alley crew has joined numerous other street performers this summer in what has become a renaissance of free entertainment in the Harvard-Brattle Square area. Well, not quite free, but for quarters and dimes, students and local residents have enjoyed an extraordinary variety of musical, magical and funny outdoor theatre since mid-June.

For years, people have brought their jokes and guitars to this part of Cambridge, making a tin can or a hat available for generous pedestrians. Over the past several months, however, the Square has become nothing short of a free-form carnival; performers attract crowds anywhere they can find a little open space, on weekday as well as weekend evenings.

One reason for the artistic explosion may be the recently opened stage between Out of Town News and Mug 'n' Muffin restaurant. Created by the new MBTA stop, the tiny macadam plaza gives performers another place to ply their trade in addition to the Brattle Square Island. Kalomymus, a bearded young man who presents his own version of the flaming batons routine, says another reason he and his cohorts have had more success recently is that the Cambridge police have refrained from breaking up larger audiences. "One cop did tell me the other night that he'd stick the torches up my ass, but I don't think he was serious, and most of them have been pretty friendly," he says.

The officers who congregate at their "sub-station" in front of the Tastee diner, around the corner from where Kalomymus performs, say they haven't changed their policy toward street entertainment, and most seemed amused, if not overjoyed, by the burgeoning crowds. "I guess anyone with a guitar can come down and make a few bucks these days," one policeman acknowledges.

According to the performers themselves, the Boston area--meaning Faneuil Hall and Harvard Squre--now rivals San Francisco as the street entertainment capital of the nation. Acts like the Back Alley troupe pass through Cambridge on cross-country jaunts "just to be able to say we've played the Square--it's quite a big deal," says Van Alstyne. Magician Peter Sosna, who insists he's the only guy in town who can perform Houdini's identity-switching act on the street, points out that some people rank New York's Washington Square Park ahead of Cambridge. "No way. They don't know what they're talking about, and you can tell because they all want to come here," he explains with a wink.

Of all the acts that turn up in this area, the Shakespeare Brothers seem to have been around the longest and receive the most credit from their colleagues for fostering the current renaissance.

"Starting a renaissance in the Square? Hell, I started THE Renaissance," insists Shakespeare Bro. Steve Aveson. "And my friend here (Allan Krulick), he started the Dark Ages. This is the big time."

Relaxing after a full evening of what they call "socio-politico-comic commentary (we denigrate all races, creeds, political groups and clothing styles)," Aveson and Krulick reflect on their six-year career on the Cambridge circuit. "We can get away with a lot of stuff here that just wouldn't work anywhere else because of generally limited vocabularies." Aveson says. "Quite true," adds his partner, "This is without a doubt the top of the gutter." Despite a penchant for one-liners and their unusual appearance--Aveson wears most of a tuxedo; Krulick sports dungarees, a baseball shirt, and a cap labeled "Shakespeare"--the duo takes performing seriously. "There are no admissions and no boundaries, so we can be funny and say things that are meaningful too," Aveson notes.

The Shakespeare Brothers do a funny schtick about being interviewed. "How much money do you make; what are your goals, and did you go to school to learn that?" Aveson asks his empty fist in a passable Walter Cronkite imitation. In fact, the money can be surprisingly good. Well-known acts that draw big crowds can pull $75 on a good evening. But like most of their colleagues, Aveson and Krulick are ambivalent about commercial success and long-term aspirations, preferring to talk about their present work. "This is a full-time job," says Aveson, "We're commited to street performing as an end in itself."

In the winters, of course, almost all of the entertainers look for more formal gigs or depend on conventional jobs. Sosna plays the college circuit and says that he prefers educated audiences because they are far easier to fool, "always looking for a complicated explanation for a trick." It's little kids he refuses to confront. "Too damed perceptive." he says.

Van Alstyne and his Back Alley partner, Marty Coffey, endorse the Shakespeare Brothers' credo of street performing for its own sake but admit in the same breath that they would like to play clubs some day. Or, they admit, they "could get fed up with the whole routine." In the meantime, they hope to save money from this summer's tour for a trip to Australia this winter and perhaps Europe next year. What they and others seem to desire more than anything else is constant variety. Coffey was once a cabinet maker, Val Alstyne "a three-piece, nine-to-five, memos-and-meetings man." The old way "just seemed so silly," Van Alstyne says.

Ammonia, a six-man electric jazz group that plays Sunday nights in the cramped front yard of the Cambridge Center for Adult Education, lugs around more equipment than many bar bands, but its members claim they have no aspirations other than "to play outside in the warm weather and entertain the people." Several contributors to the smooth Ammonia sound are professional musicians, and one notes that the group has potential "because we've already fought it out and slugged each other around, so we know how to survive." For now though, they're willing to run their cords all over Brattle St. and play vigorous two-hour sets for the $30 or $40 left in a guitar case, which they split six ways.

Perhaps the only disheartening aspect of the summer entertainment scene has been the apparent retirement of Brother Blue, a legendary singer of songs and teller of tall tales. After a few appearances early in the season, he all but dropped out of sight. Wherever he is, however, he's probably pleased that so many have followed his example in bringing life to the city's street corners.