One participant in the demonstration that stopped San Francisco traffic cold last Friday describes some of the factors that turned participants from innocence to anger in a short time. While the group never conformed fully with city rules, there was an air of joyful following at first; but patronizing talk by city officials and a violent police response may have planted the seeds of a conscious movement. Michael Blanding, a freelance writer, bicycles to and from work.

The Critical Mass demonstration that snarled San Francisco commuters for hours last Friday started out far more mass than critical. Whatever the organizers had in mind, this did not begin as a political protest, nor as a determined effort to change city transportation policy. Indeed, it started like one big bicycle-lovers' joy ride -- but the solidarity of numbers and a brutal police response may have transformed it into a movement.

A mob mentality ruled from the first, as thousands of cyclists veered off the city-approved route to barrel down the center of Market Street. It is clear that many of the cyclists had no idea where they were going or why, let alone knowledge of the approved route. Soon the entire street was visibly jammed from the waterfront to Van Ness Avenue, a two-mile stretch bordering San Francisco's downtown, with bewildered buses snarled in the fray.

It was like a party on wheels. Grins were on every face, and howls of elation echoed off the skyscrapers. Many drivers honked their car horns in solidarity, and pedestrians lining the sidewalks were cheering and offering high fives. Most commuters, however, stared staring silently ahead, and some shook fists -- though far fewer than you might think.

But the spirit began to change when the pack reached the first exit ramp for the Bay Bridge and found it blocked by San Francisco Police. Someone raised his bike in the air, wheels still spinning, the frame a dark silhouette in the light that sneaked under the overpass. Soon hundreds of others followed suit, holding their bikes in the air, their screams echoing off the steel beams above.

Here were bicyclists, used to being squeezed into curbstones, standing proud, feeling part of something large enough to own the streets.

Talking to the riders beforehand one got no inkling of this possibility. Sure, people were handing out alternate route maps, and there was plenty of defiant rhetoric, mostly from people at the back of the pack. But most people just wanted to follow the leader and have fun -- "like sheep" they joked.

It wasn't until police started issuing threats through bullhorns, about how people who veered from the route would be "dealt with," that the boos started to escalate, and the cries of "Let's Go!" began to seem reasonable. And when the mayor got up and started talking about "fun" and "bicycle friendly" cities, it was just too much. Thousands of people who wanted only to be recognized -- after demonstrating once a month for five years -- were being threatened and patronized instead.

That was enough to start the mob. With the mysterious mob intelligence, cyclists rode where they'd be most visible. Critical Mass has no leaders. At each intersection, those riding in front broke off to block traffic, while those just behind them went wherever they felt like going.

The result was sheer bedlam. The police chief warned of arrests, but few riders could have heard it. As I watched, police moved to arrest a fortyish gray-bearded cyclist for running a red light. They were immediately surrounded, and police started going after people in the crowd. Later, I heard an enormous crash and watched as a police van collapsed on a punctured tire.

The finale made as little sense as anything. Police decided to cart away all the people on a block selected at random. Witnesses told of an ugly scene with police cycles ramming bikes, billy clubs used to knock people off their bikes and smash cameras.

No doubt, there will be speculation about who to blame for all this. But it doesn't matter. What does matter is how the city responds now that the bike riders are truly visible. Because leaders or no leaders, a movement has begun. The "Sheep" -- like the man who was just riding to the gym and decided to join and have a little fun -- were bewildered, angry, defiant. Turned into wolves.