SAN FRANCISCO 

The hour of the night skaters was upon the city.

Valkyries in Velcro, speed demons in spandex, nearly 300 strong, swooped through the roller coaster streets of the twilight, town, claiming them as their own.

Tourists turned, startled at the nocturnal vision bearing down upon them and then gone in a flash, cries of "we're rolling!" echoing in its wake. Motorists sat helplessly and waited as the horde swept by, its members bobbing and weaving like erratic moths, some glowing in fluorescent green neck-rings.

The police did not interfere in the Friday night ritual; officially, they say they have more important things to do than ticket skaters, but one skater offered a different spin: "We are not just In San Francisco. We are San Francisco."

The night skaters, in contrast, say they are only after pure fun, with nothing to prove except that in-line skating, one of the fastest-growing sports in the United States, feels good. But under the Californian imperative that recreation equals identification, it Is serious fun, indeed.

Other bladers get together for group skates in other cities, but as far as many skating enthusiasts can tell, the 300,400, sometimes as many as 600 "Midnight Rollers" who take to the streets here on Friday nights constitute the biggest such group In the country.

"What it's really all about is, it's a community," said David G. Miles Jr., father of the Friday night tradition and founder of CORA, the California Outdoor Roller-Skating Association. "It's a community of people who if not for skating would never have met, and the skating is the glue that keeps everybody together."

Dan Filner, a regular shimmering in a cape of green reflecting sequins, said skating can have transformational powers, too. "It really changed my life," Mr. Filner said. "I went from being a computer nerd to being an Outgoing, physically active per-son. I'll live longer - If I don't get run over."

The group's genesis dates ~ the 1989 Loma-Prieta earthquake here. Earthquake damage forced the closure of the Embarcadero Freeway, and a few enterprising skaters - Mr. Miles among them - stole onto it one night and found themselves in heaven, rolling along smooth, empty expanses for miles and miles. The freeway was eventually torn down, but the nucleus of a regular skaters' group had been formed.

Six-odd years later they have evolved into this, a tradition so powerful that some skaters drive hours to attend, a mix of masters with concrete quadriceps and neophytes advised to bring cab fare in case they don't make the 12.5-mile route. The group is too freewheeling to impose dogmatic rules, but Mr. Miles begs skaters to wear protective gear -helmets, knee-pads and reflectors.

The night begins next to a blaring boom-box in a parking lot next to the Ferry Building, as skaters dance, warm up, hob-knob and once-over each other.

With a cry of "Let's gooooo!" from Mr. Miles - who's day job is as a skate instructor, the group took off last Friday night, streaming along the Embarcadero, which runs along the waterfront, and through Fisherman's Wharf. Then it was on to the Marina and to the stately Palace of Fine Arts, where they formed a giant whirling circle under its dome. After a mass stop for fuel at a designated grocery store, the group went on, swarming through narrow Fillmore and Union Streets and getting badly on the nerves of at least one driver:

A tunnel under Broadway provided the scariest segment, a long ride along a narrow walkway whose surface filled the tunnel with the chunga-chunga-chunga of skates and made skaters' teeth chatter. Then they were on to Chinatown, a long stop for horseplay in Union Square, back to the Ferry Building and hor to recuperate, at about 11:30 P.M.

For true addicts, the next fix wa' only two days away, when they wot gather for a traditional Sunday of skating in Golden Gate Park, and there are more demanding events, including a marathon skate to Los Angeles. But for many, the nightskate is special.