Citywide tactical alert as Occupy LA protesters skirmish with riot police A unidentified man is arrested by Los Angeles police on Fourth Street and Spring Street in downtown Los Angeles late Thursday. Los Angeles was placed on a citywide tactical alert Thursday night when hundreds of people, including Occupy LA protesters, gathered near downtown LA as part of a demonstration by the group that set up camp on the City Hall lawn late last year. Shortly after 11 p.m. ET Friday), police ordered the crowd to disperse from the area. "This kind of took us by surprise, we were not expecting them to show up tonight," said Officer Karen Rayner with LAPD. The demonstration started at about 8:40 p.m. 11:40 p.m. Thursday, when protesters began taking over the intersection of Fifth and Spring streets, Rayner said. Read the full story on NBC Los Angeles The demonstration appeared to be linked to an Occupy LA event scheduled for Thursday night. Notices appear online The protest group posted the following notice about the demonstration on their Facebook page Thursday afternoon: Tonight, #ArtWalk in #DTLA becomes #ChalkWalk! Occupy Los Angeles has had a laughably ridiculous 12 arrests the past 6 weeks for children's sidewalk chalk. Tonight from 7-9pm, occupiers, artists, enthusiasts, rebels, and the intrigued will defend the First Amendment and freedom of speech. Dozens of people were estimated to have been arrested in the incident, Rayner said, adding that it was too early to know exactly how many people had been taken into custody or why. There were reports of injured LAPD officers, but officials had yet to confirm that information. During the first hours of the demonstration, the street -- which was dotted in chalk markings -- was flanked by skirmish lines of riot police. At times, crowds and police could be seen running from the area. PhotoBlog: Occupy Oakland protesters attempt port shutdown Messages written in chalk covered the downtown intersection. "You wouldn't shoot your kids for this, why shoot us?" and "Chalk the police" were among the notes. Sam Mircovich / Reuters Los Angeles Police officers hold their position in Downtown Los Angeles, California late Thursday. Police made several arrests of protestors who took over the monthly art walk in the area with their own chalk graffiti. A man says he was visiting ArtWalk when he was struck in the ribs with a rubber bullet. I was walking down the street and I saw a group of people. I was just here for ArtWalk, I didn't know anything was gonna happen," Charlie Shepherd said. Skirmishes Aerial views of the scene showed demonstrators throwing objects at police. A member of the crowd threw a traffic cone at an unmarked police car driving by and some people could be seen dancing in front of police lines. A few skirmishes appeared to break out as the police attempted to move the crowd from the area. During the disperal orders, police told media outlets to stay at least 40 feet away from the standoff between the crowd and officers in riot gear. 'Overwhelming military-type response': Report criticizes Oakland police handling of Occupy protests The nationwide Occupy movement -- whose message was included several ideas and did not have a designated media liaison -- grew out of contempt of what members called corporate greed and the nation's severe budget slashing which typically targeted education and social services. Occupy LA set up its campsite outside City Hall for about two months last fall before some 1,400 police officers swept demonstrators off the lawn. As many as 500 men, women and children were estimated to participated in the encampment. The LA city council estimates the costs related to Occupy LA could reach $5 million for policing, cleanup and other costs.