A nurses' union picketed Chicago Mayor Rahm Emanuel's office on Monday after three of its members were arrested at an Occupy Chicago protest on Saturday.

The police arrested two nurses and a union organizer, along with 130 Occupy Chicago protesters who refused to leave Grant Park. But the union said its members were only there to provide medical care to the protesters, not to join in the protest themselves, and it demanded that the police drop the misdemeanor trespassing charges against the nurses.

It was the wrong move, RoseAnn DeMoro, the executive director of National Nurses United, the largest union of registered nurses in the country, said of the arrests. We were there to make sure if the occupiers get harmed, they have first aid.

But if the union wasn't interested in joining the protests before, it is now, DeMoro said.

The nurses are angry, and it's made us double our resolve, she said, adding that members of National Nurses United have set up first aid tents at a number of Occupy protests -- including Occupy Wall Street in New York and offshoots in Los Angeles and San Francisco -- and the Chicago police are the only ones who have arrested nurses.

So the union decided to protest outside Emanuel's office, and the Occupy Chicago protesters voted on Sunday to join them.

This is an attack on the core of what it means to be an American citizen, one protester, Patrick Robinson, 24, told The Chicago Tribune. We condemn [Emanuel's] gross violation of the human rights of those arrested.

The police department released a statement saying that officers had dealt with the protesters appropriately.

The Chicago Police Department works diligently to safeguard the constitutional rights of all persons, the statement said. Members of the Chicago Police Department uphold the highest standards of police conduct, and this extends toward individuals in the exercise of their First Amendment rights.

According to the police, most of the 130 people arrested on Saturday have since been released on bail.

Occupy Chicago is one of numerous Occupy protests that have sprung up nationwide since protesters began occupying Zuccotti Park in Lower Manhattan last month. There are also ongoing protests in Atlanta, Boston, Houston, Indianapolis, Los Angeles, Nashville, Pittsburgh, San Francisco, St. Louis and various other cities.