alt Hacker NewsFacial recognition could be used to identify people who are likely to participate in protests and prevent them from doing so in advance by intercepting them at their boarding station or somewhere near it. You could also scan for persons arriving in a city where a protest might be plannend and arrest them beforehand.
As with your sibling comment, what you’re describing is theoretically possible, but it’s also more difficult and resource intensive than the simple dumb alternative of blocking off the area of the planned protest and putting some police officers outside nearby tube stations to arrest anyone who’s clearly intending to head to the protest.
It’s already easy for the police to prevent protests in a practical sense. It’s a political and legal issue more than a technical one.