LOS ANGELES — Walk into the Museum of Contemporary Art here and urban graffiti — scrawled on walls, buses, a subway car and behind glass — is being celebrated by this city’s art lovers in a hugely popular show. Walk a block away and the same sort of scrawling could get you thrown in jail.

An exhibition of street art that opened last week has been responsible, the authorities say, for a new wave of graffiti on buildings, lampposts and mailboxes in downtown Los Angeles, forcing a fresh crackdown on an activity that the police thought they had brought under control. And it has put them in the awkward position of trying to arrest people for doing something that is being celebrated by the city’s cultural establishment.

On another level, the exhibition, “Art in the Streets,” has fueled a 40-year dispute about the nature of graffiti and the appropriateness of a mainstream arts institution, like the Geffen Contemporary wing of MOCA, conveying legitimacy on an activity that some people see as nothing short of vandalism.