What are your rights at a protest?
The rights to freedom of expression and freedom of assembly are protected, and they are qualified rather than absolute — meaning they can be restricted where the law permits and the restriction is proportionate. Understanding where those limits sit is what the question turns on.
What the rights cover. Peaceful protest, including protest that is disruptive, offensive or annoying — protection is not limited to views people agree with, which is the point of having the right at all.
What police can lawfully do:
Impose conditions on marches and static assemblies — on route, location, duration and numbers — where specified statutory tests are met.
Require notice of a procession, usually six clear days in advance, though spontaneous protest is not automatically unlawful.
Use dispersal, stop and search and arrest powers where the legal grounds exist.
Contain a crowd in defined circumstances, which has been litigated extensively and is lawful only within limits.
What has changed. Public order legislation has expanded police powers in recent years, including in relation to noise, locking on, tunnelling, obstructing highways and interfering with national infrastructure, and has introduced new offences and orders. These changes have been contested in the courts and by human rights bodies, and some provisions have been successfully challenged — so the position is genuinely unsettled rather than fixed.
What you are entitled to if stopped or arrested:
To ask the legal power being used and the grounds.
To be told the reason for arrest, at the time.
Free legal advice at the police station, at any time, and independent of the police.
To say nothing, with the caution explaining that inferences may be drawn in defined circumstances.
To have someone informed of your detention.
Practical points: write a solicitor's number on your arm rather than relying on a phone; know that filming police in public is generally lawful; bring identification if you are willing to be identified, and note that you are generally not required to give your name for a stop and search; consider legal observers; and be aware that arrest without charge can still mean hours in custody.
General information, not legal advice.