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By Erik Ortiz and Emmanuelle Saliba, Euronews

Rival ad agencies in opposing New York City office buildings are trying to stick it to each other — in the form of Post-its.

Sticky notes arranged in colorful patterns or spelling out messages have covered window fronts along lower Manhattan's Canal Street. The unusual battle — chiefly between ad firms but since spreading to other companies in the buildings — has prompted passersby to take notice of the Post-it tit-for-tat.

The first message was spotted last week when someone at the firm Horizon Media wrote, "Hi," according to reports.

Creatives from Sandbox Studio and its parent company Creative Drive put up a Marilyn Post-it yesterday afternoon. It took only about one hour to put up the piece and one hour of prep.Sandbox Studio / Creative Drive

Then, an employee at Havas Media shot back: "Sup."

And from there, the window wars heated up, spawning the hashtags #CanalNotes and #PostItWars — and inspiring office workers around the world to rise up from their cubicles and mount similar challenges.

The staff of Sandbox Studio and its parent company, Creative Drive, put up a Marilyn Monroe-inspired Post-it Tuesday from its office windows in Manhattan.

"We are a photo studio, so we wanted to do something more creative, which was why we went with a piece of art," employee Andrea Raymer told NBC News.