Ask the Declaration → Your rights, by who you are
The right to gather and object to your government is not one clause but several, and it starts with the very first words of the Bill of Rights.
Descriptive, not legal advice · the founding documents' own words
Freedom of speech, of the press, to assemble peaceably, and to petition the government for a redress of grievances. The core of the right to protest, all in one sentence.
Protection against unreasonable searches and seizures, which governs when police may stop, search, or detain you at a demonstration.
Due process and equal protection, which extend the First Amendment's limits to state and local governments, not just Congress.