The right to criticize police, even through vulgarities and rude gestures, is firmly enshrined in the First Amendment. As Supreme Court Justice William J. Brennan Jr. wrote in 1987, in a ruling striking down a Houston ordinance that made it unlawful to oppose or interrupt a police officer, "The freedom of individuals verbally to oppose or challenge police action without thereby risking arrest is one of the principal characteristics by which we distinguish a free nation from a police state."

https://caselaw.findlaw.com/court/us-supreme-court/482/451.html