Close to 20 businesses were damaged after what started as a peaceful immigrants' rights march in downtown Santa Cruz turned violent, requiring police to call other agencies for help, authorities said.

Police spokesman Zach Friend said an estimated 250 people started marching through the city around 10:30 p.m. Saturday.

It was a harmonious but "unpermitted and unsanctioned event," he said, until some in the crowd started breaking windows and spraying paint on retail shops that line the downtown corridor.

Friend said he wasn't sure if the damage was caused by people marching in support of immigrants' rights, or if the group was "infiltrated by anarchists."

Anarchy signs were spray-painted on some of the buildings.

"They're a group of people who seem to fancy themselves as revolutionaries, but what they really are are a group of morons," Friend said.

A fire was started in a coffee shop entryway but was extinguished once police cleared the way for firefighters, Friend said.

Eighteen businesses were damaged, with the cost of repairs estimated at between $50,000 and $100,000. No injuries were reported.

Once officers arrived from other agencies, police were able to restore order in about 20 minutes, Friend said.

One person was arrested on suspicion of committing vandalism, though police expect to make additional arrests once investigators have a chance to gather video of the event.

"I assure you, more people will be held accountable for this as we review videos," Friend said.