LAPD officers in riot gear broke up a crowd of protesters who filled downtown streets during the monthly ArtWalk, ending with 17 arrests and four police officers injured, authorities said.

The group started filling streets about 8:40 p.m. Thursday, prompting about 140 police to respond to the scene. A police spokesperson said police believe at least some of the protesters, who may have been involved in a rogue event known as "Chalk Walk," were associated with the Occupy Los Angeles movement, but that has not been confirmed.

LAPD spokeswoman Norma Eisenman says people were arrested for multiple reasons, including vandalism for using chalk on the sidewalks. One person was arrested on suspicion of felony vandalism and nine for misdemeanor vandalism, City News Service reported. 

“They were vandalizing the sidewalk and privately owned buildings writing in chalk. The city attorney – this is something they prosecute. It is a misdemeanor and sometimes it can be a felony,” Eisenman told KPCC. The introduction message on Chalk Walk's Facebook page maintains that chalking the sidewalk is not a crime.

Los Angeles Police Department spokeswoman Karen Rayner said some protesters began throwing rocks, bottles and other objects at officers, and police were on citywide tactical alert. The crowd persisted for a couple hours, and police dispersed the crowd before midnight. Three male officers were treated for minor injuries, one female officer was treated for a minor concussion.

The L.A. Times reported that a woman who identified herself as part of the Occupy movement said protesters attended the monthly event with the intention of showing support for people previously arrested for chalking on the sidewalk.

"We were handing out free chalk for freedom of speech," said Cheryl Aichele, 34.

In a statement that City News Service cited, Aichele said most of those arrested had been chalking the sidewalk around Fifth and Sixth streets and most were not members of Occupy L.A.

"Occupiers claim the LAPD started the violence and say if they were allowed to chalk, that none of the subsequent actions would have happened,'' she said.

There's much more about the scene Thursday night over at Blogdowntown.

This post has been updated to reflect new arrest numbers. Police had told KPCC that 20 people were arrested.