I think I had the same argument with my 4-year-old the other day.

LOS ANGELES (KTLA) — Demonstrators threw rocks and bottles at LAPD officers as police and protesters squared off in downtown Los Angeles late Thursday night.

At least 20 arrests were made and four officers were injured.

The confrontation began after the Occupy L.A. movement urged supporters to gather at a monthly event called ArtWalk. Demonstrators arrived to protest the apparent arrest of people who had written on the sidewalk with chalk during previous events.

The movement advertised the “Chalk Walk” event on its Twitter feed, saying, “we will be handing out free chalk for free speech.”

Shortly before 10 p.m., the LAPD issued a tactical alert, and sent hundreds of officers to the demonstration at Spring and 5th streets.

As the crowd of protesters and the number of officers grew, one demonstrator threw a bottle over the crowd and hit police in the head.

Protesters allied with the Occupy movement chanted, “Whose streets? Our streets!” and yelling expletives about police, according to witnesses.

Occupy said on its Twitter feed that police were shooting rubber bullets at people, and posted a picture of a demonstrator hit by a bullet.