Nina De Chiffre, 20, of Milan, was at the protest against controversial plans for a new high-speed TAV train line in November when she kissed riot officer Salvatore Piccione's helmet, TheLocal.it reported.
"I wanted to make fun of them [the police], and I would say that we were successful," De Chiffre told La Stampa.
Now, De Chiffre is facing charges of sexual violence and offense to a public official.
Franco Maccari, the secretary-general of the COISP police union, told Radio24 that if the police officer had kissed a protester, "World War Three would have broken out."
"We're tired of taking blows from the No TAV group -- and not just from them. Today, protesting is a sacred right, but when they overstep the line of legality, there can be no flexibility," Maccari said.
Meanwhile, Piccione said: "When I'm wearing my uniform I represent the police institution, and I have an obligation not to react to provocation. The important thing is that, in the end, the demonstration went ahead without disturbance."