When I got out of the subway at Washington Square on Thursday afternoon, there were sirens blaring, cop cars driving the wrong way down Sixth Avenue, and lots of people carrying signs and chanting. After gathering in Union Square as part of a “Day of Action” to mark the two-month anniversary of Occupy Wall Street, thousands of college students and union members were traipsing through the Village en route to Foley Square. Why they were going down Sixth Avenue rather than Fifth or Broadway, I have no idea, and neither did the cops. One of the marchers said it had been too chaotic on Broadway, so they had headed west.

The scene was noisy but good-humored. As the crowd moved south, cops ran alongside them shouting, “Stay on the sidewalk.” Below Bleecker, some of the kitchen staff from Da Silvano came outside to see what the commotion was. At the corner of Spring Street, a young O.W.S.-type in a hoodie ignored the cops’ orders and quickly found himself pressed against a catering truck with his wrists cuffed. “Shame, shame,” other protestors shouted at the cops. As the cops put the kid in the back a squad car and drove him off, I thought back to the nineteen-nineties, when I lived in Soho for nearly ten years. In all that time, I don’t think I spotted a single political protester. The only things I ever saw anybody getting arrested for were shoplifting and three-card monte.

The demonstrators moved on, turning left on Canal and right on Broadway. Seeing a tall, handsome guy with a black flag, black flowing hair, and two pretty young women accompanying him, I fell in behind him for a bit, thinking I might have happened across a latter-day Buenaventura Durruti. Near Walker Street, the anarchist’s female followers abandoned him for Starbucks, leaving him to march on alone. At Foley Square, he disappeared into a huge crowd, which the N.Y.P.D. estimated at more than twenty thousand. I don’t know where these figures come from, but the Square stretches for several blocks and it was packed to overflowing.

Up near the front, at the corner of Reade and Centre Streets, I came across a group of about twenty female health-care workers from Local 1199-S.E.I.U., who were standing beneath a banner that said “Jobs Not Cuts.” They were jumping up and down singing: “Everywhere we go-o. People Want to Know-o. Who we are. Who we are. So we tell them. We are the union! We are the union!” Although most media reports focused on the sizable O.W.S. contingent, this was overwhelmingly a union crowd. There were public-school teachers, teamsters, transit employees, Verizon workers, CUNY staffers, and many others.

In New York, at least, it was support from unions that transformed Occupy Wall Street from a ragtag protest movement of activists, agitators, and students into something broader: an embryonic Tea Party of the left. When the mayor threatened to close Zuccotti Park one morning last month, many of the people who rushed down there to foil his efforts were union members. Now that Bloomberg has succeeded in his goal, the question of how to preserve and enlarge the protest movement looms large.

Many outsiders who sympathize with O.W.S. are secretly (and not-so-secretly) relieved that Bloomberg did what he did. With the camps gone, they argue, the movement can concentrate on more important things, such as campaigning for higher taxes on the rich and tougher financial reforms, and ensuring the reëlection of a Democratic President. Even Adbusters, the Canadian group that organized the original O.W.S. occupation, had appeared to advocate a strategy of declaring victory and going into hibernation for the winter.

Initially, I was thinking along similar lines. Now, I’m having some second thoughts. To be sure, there was something a little absurd about piling into a concrete plaza in lower Manhattan and expecting to stay there indefinitely. But the events unfolding in Zuccotti Park provided a focal point, real and symbolic, which supporters across the country could rally around and mimic. In a media-addled society that rarely focuses on one story for more than a few days, the occupation also provided an ongoing narrative that people could follow in real time. (Before setting out to yesterday’s march, I spent a couple of hours watching it on a live-stream provided by “The Other 99.”)

Last year, when Republicans in Congress were trying to torpedo the Volcker Rule, which placed tight limits on proprietary trading by big banks, I spoke with Barney Frank, who was then chairman of the House Financial Services Committee. This is what he said: Public opinion and the people can overcome the vested interests, but they have to be mobilized. If they aren’t mobilized, the vested interests can win out.

However it happened—and I don’t think even the organizers know the answer—the very act of occupying Zuccotti Park unleashed a nationwide political mobilization the likes of which hasn’t been seen in a long time. If the encampments disappear and everybody goes home, how can this mobilization be sustained? And without the mass mobilization, what are the prospects of any real political change?

At about six o’clock, the protestors moved south towards City Hall and the Brooklyn Bridge, which they were intending to walk across (on the footpath). It was cold and dark, and a few young agitators near me were baiting the cops, unhinging the barricades they had set up and knocking them over. After the violent clashes earlier in the day, near Wall Street, I thought more trouble might brewing, but it didn’t materialize. The cops kept their cool, and the troublemakers got swept along in the huge, peaceful crowd, which was chanting, over and over again: “All day, all week: Occupy Wall Street.”

Photograph by Spencer Platt/Getty Images.