Answer:
Citizens have the right to protest against something they believe is wrong, and if petitioning doesn't work, they should organize a protest anyway. Other people, however, have the right not to participate if they don't want to, and no one can obstruct traffic or prevent people from entering or leaving any place; and obviously, third parties' properties are theirs and should be left alone. The police can intervene and even arrest and charge violators in this circumstance if the leaders cannot restrain individual demonstrators who are abusive for whatever reason.
They are essentially exercising their right to petition and speak freely under the first amendment. The police have the authority to protect the rights of the general public under the first amendment.