The arrest of an organizer of an anti-war gathering yesterday at Union Square that featured Cindy Sheehan provoked a furious reaction from protesters, who pressed up against police officers and called them “pigs,” “fascists,” and “Stalinists.”

The commotion overshadowed the visit to the park by Ms. Sheehan, the prominent anti-war activist whose son was killed in action in Iraq last year.

After she delivered a brief speech before a couple of hundred people at the south end of the park, New York City police officers seized the sound equipment used in the event and arrested one organizer, Paul Zulkowitz, charging him with unlawful use of a sound device and disorderly conduct.

After Mr. Zulkowitz was hauled away, protesters hurled insults at the police, who formed a corridor along the southeast entrance into the park. A sarcastic trumpeter played “The Star-Spangled Banner.” By that point, Ms. Sheehan had been whisked away and was headed for a television interview.

An argument also broke out between two protesters over the wisdom of shouting at the police. Lorcan Otway, a Quaker, said he was dismayed by the arrest but said, “Yelling at the cops isn’t going to do any good.” Richard Degen, a self-described anarchist, told Mr. Otway he was acting weak in the face of an injustice.

Police said the organizers, a coalition of anti-war groups, did not obtain a sound permit for the event, which is required to use amplifiers for gatherings in public parks. Police were heard warning the organizers to turn off the sound equipment.

A spokesman for the parks department said organizers had a permit for tabling but not for a demonstration. It was the third time Mr. Zulkowitz has been arrested in the past six months. His most recent arrest was on August 29 at Union Square, for leading a similar demonstration without a proper permit.

For more than a month, anti-war groups, including the Green Party and United for Peace and Justice, have set up what they call Camp Casey NYC at Union Square, with a tent and tables piled with leaflets. Named after Ms. Sheehan’s son who died in the Iraq war, the original Camp Casey was Ms. Sheehan’s campsite outside of President Bush’s ranch in Crawford, Texas.

Ms. Sheehan’s stop in New York City is part of her nationwide speaking tour that began in late August. She is to arrive in Washington, D.C., tomorrow and is expected to participate in a rally on the National Mall on September 24.