NEW YORK - The Occupy Wall Street movement marked its six-month anniversary with demonstrations and marches in New York City on Saturday night, but police moved in and arrested dozens when protesters tried to set up camp in Zuccotti Park, the place where it was born.

Some demonstrators locked arms and sat down in the middle of the park in the City’s financial district after police announced that the park was closed. Officers then poured into the park, forcing most of the crowd out and surrounding a small group that stayed behind. Police formed a human ring around the park to keep protesters out.

Several people were arrested, police said, but no numbers were given. An unused public transit bus was brought in to cart away about a dozen demonstrators in plastic handcuffs. One female under arrest had difficulty in breathing and was taken away in an ambulance for treatment.

For hours, the demonstrators had been chanting and holding impromptu meetings in the park to celebrate the anniversary of the movement that has brought attention to economic inequality, as police mainly kept their distance.

But police said the tipping point came when the protesters started breaking the park rules. “They set up tents. They had sleeping bags,” he said. Many protesters, who were told to leave the park, shouted and officers took out their batons after a demonstrator threw a glass bottle at the bus that police were using to detain protesters.

Sandra Nurse, a member of Occupy’s direct action working group, said police treated demonstrators roughly and made arbitrary arrests. She disputed the police assertion that demonstrators had broken park rules by putting up tents or getting out sleeping bags.

As they have in past marches, protesters led police on a series of cat-and-mouse chases.

Marchers at the front of the crowd would suddenly turn down narrow-side streets, startling tourists and forcing police to send officers on motor-scooters to contain the crowd.

The movement has made headlines for its clashes with police after campsites were set up for months in cities from New York to California. The camps were eventually shut down by authorities citing zoning regulations and public health concerns.

In New York, the Occupy movement lost significant momentum in November when a pre-dawn sweep broke up the encampment at Zuccotti, although Occupy protests in Oakland, California, in January led to police firing tear gas into crowds of protesters and more than 200 were arrested.