When members of the loose protest movement known as Occupy Wall Street began a march from the financial district to Union Square on Saturday, the participants seemed relatively harmless, even as they were breaking the law by marching in the street without a permit.

But to the New York Police Department, the protesters represented something else: a visible example of lawlessness akin to that which had resulted in destruction and violence at other anticapitalist demonstrations, like the Group of 20 economic summit meeting in London in 2009 and the World Trade Organization meeting in Seattle in 1999.

The Police Department’s concerns came up against a perhaps milder reality on Saturday, when their efforts to maintain crowd control suddenly escalated: protesters were corralled by police officers who put up orange mesh netting; the police forcibly arrested some participants; and a deputy inspector used pepper spray on four women who were on the sidewalk, behind the orange netting.

The police’s actions suggested the flip side of a force trained to fight terrorism, in a city whose police commissioner acknowledges the ownership of a gun big enough to take down a plane, but that may appear less nimble in dealing with the likes of the Wall Street protesters. So even as the members of Occupy Wall Street seem unorganized and, at times, uninformed, their continued presence creates a vexing problem for the Police Department.