LOS ANGELES – The chalk scrawls began showing up in downtown Los Angeles in May.

Members of the Occupy L.A. movement, with the support of some homeless rights advocates, used pastel chalk to express their anger about gentrification in downtown and how it was pushing the poor out.

For weeks, the chalk protests were little noticed, and Los Angeles police quietly began arresting the so-called chalkers on vandalism charges.

But on Thursday, a small group of activists gathered at the corner of Fifth and Spring streets during the heart of downtown’s popular monthly ArtWalk, handing out chalk to passersby and using the sidewalks as their canvas for more anti-gentrification and some anti-police slogans.

For reasons that are not entirely clear, clashes began between some activists and Los Angeles Police Department officers, resulting in a melee that left four officers injured and more than 15 people arrested. The LAPD called a citywide tactical alert, streamed hundreds of officers into downtown and used rubber bullets to disperse the crowd.

The incident highlights long-simmering tensions as downtown L.A.’s major revitalization has ushered in thousands of new residents as well as upscale lofts, restaurants and galleries to the city core. The gentrification has transformed large swaths of downtown, but it has spread into areas that were traditionally home to some of L.A.’s poorest residents.

Occupy L.A. has made this a central focus as it attempts to rebound since the group’s eviction from City Hall park last year. Both Occupy and Los Angeles Community Action Network, a homeless and low-income advocacy group, have targeted the Central City Association, the leading downtown business organization, with their protests.