We have the right to come together with others and peacefully express our views. Authorities must allow us to take part in marches, protests and demonstrations.
Article 11 of the ECHR is closely linked to freedom of expression as it applies to protests, marches and demonstrations, counter-demonstrations, press conferences, public and private meetings and more – but it does not protect intentionally violent protest.
People turn to Article 11 when public authorities either stop a demonstration going ahead, take steps in advance to disrupt a demonstration or store personal information on those taking part.
The State can’t interfere with your right to protest just because it disagrees with protesters’ views, because it’s likely to be inconvenient and cause a nuisance or because there might be tension and heated exchange between opposing groups.
Instead it must take reasonable steps to enable you to protest and to protect participants in peaceful demonstrations from disruption by others. (Liberty)
The powers the police are using is covered by the Public Order Act which has been in use since 1986, however, where stop and search was only legally done where the police suspected acts of terrorism etc, it can now be carried out on any peaceful protester.
That is the way authoritarian governments behave.
The onus is no longer on the police when they stop people going about their lawful business. They can intimidate a perfectly legal protester by stopping and searching without any explanation.