What a Mass-hole!

A former Boston prosecutor on a crusade against the NYPD was arrested for vandalizing City Hall — which he boasted about on his blog, sources told The Post.

Robert Constantino, 34, was busted four days after surveillance tape captured him stenciling sets of red hands and the words, “NYPD Get Your Hands Off Me” on the East Gate and another pillar.

The lawyer, who works for a downtown think tank, began his self-indulgent spree Monday in Brownsville — where his lecture on the city’s stop-and-frisk policy was not well received.

“How about you go the f–k back to Manhattan before I f–k you up?” a teen told him, according to his blog.

The activist, who lives in Crown Heights, then went to Manhattan and stared at the Wall Street bull, he said.

He staked it out for five hours, but gave up because cops “had that thing guarded like Fort Knox.”

Constantino settled for spraying his message on pillars at City Hall around 3 a.m. Tuesday — hoping to get caught red-handed.

“I bent over and picked up my things right as a police car, taxi, taxi and police go-cart thing went by, all without somehow noticing that I was surrounded by red graffiti and holding a stencil covered in red paint.”

He even wore a Hugo Boss suit, an Yves Saint Laurent tie and Cole Haan wingtip shoes to look good for court. But he wasn’t arrested until Friday.