LOS ANGELES (AP) — A free-speech protest turned into a bottle-throwing clash with police that injured four officers and ended with 20 arrests, authorities said Friday.

The confrontation occurred Thursday night during Art Walk, a monthly event that encourages people to visit galleries, bars and restaurants in a revitalized downtown area.

Protesters said they were demonstrating against the recent arrests of some people for chalking on sidewalks.

"We were handing out free chalk for freedom of speech," Cheryl Aichele, 34, of the group Occupy L.A. told the Los Angeles Times.

The demonstrators were ordered to disperse shortly before 10 p.m. after they began writing on the sidewalks and buildings with chalk, Officer Norma Eisenman said.

Some messages chalked at the intersection of Spring and 5th streets included "May the youth rise" and "End the Fed," the Times reported.

Instead of dispersing, police said some protesters began hurling cans and glass bottles. Three officers were treated for minor injuries but a policewoman received a mild concussion when she was struck in the helmet, Eisenman said.

Several protesters reportedly were hurt as police struck with beanbag ammunition and batons. Eisenman, however, said she had no information about protester injuries.

A citywide tactical alert was called and about 140 officers responded, Eisenman said.

Officers in helmets and face shields formed skirmish lines and pushed the crowds out of the area, dispersing the protest in about 90 minutes.

Police arrested 20 people, 10 on suspicion of vandalism and the rest for crimes ranging from blocking a roadway to assault with a deadly weapon on a police officer, Eisenman said.

They could face criminal charges, she said.