"In the months leading up to the wedding, a series of demonstrations by students, trade unionists and others saw hundreds of thousands of people take to the streets to protest against government cuts. There were outbreaks of disorder and in one case student demonstrators attacked a car carrying Prince Charles and his wife Camilla." [1]

I might just be being cynical but this sounds like an intelligence led operation to remove people who were at a high risk of commiting public disorder. It seems pretty limited that it was 20 people and they were picked up all across london - the police didn't just swoop in on a group of protesters with anti-monarchy banners, they were specifically targeted. I appreciate although the UK has no "constitution" we have to free protest but from the riots last year I think The Met have every right to be cautious about preventing demonstrations which could spark into something dangerous at one of the biggest public spectacles of the decade, when the entire crowd is made up of families trying to enjoy themselves.

If the cause is so noble, one shouldn't need to use a large public spectacle as a vehicle for it.

[1]http://www.guardian.co.uk/uk/2012/may/27/activists-arrested-...