Such an instance of free speech is clearly not prosecutable and clearly is protected speech. This is clearly demonstrated by the mere fact that the protests have been occurring there every single day since September. Yet no one in nearly four months of five days a week protests has been arrested for merely being there protesting and calling them prutzas. That is because that is not an arrestable offense. It is protected speech and democratic protest. The chilonim would have loved nothing more than to arrest frum Jews otherwise. There is no law against it though. It’s called Democracy.
(The point about defamation being a civil, and not a criminal, matter was relevant to your mistaken notion that one can be arrested for defamation or libel. Parenthetically, in most democracies other than the U.K., the burden of proof to a claim of defamation lies with the allegedly defamed party and not with the accused.)