It’s a difficult one for the police, stop it happening ( really? What, use water cannon on black protesters)you cannot stop thousands from gathering if that is their will, not without violence.
There are those who genuinely want to protest at what happened ( and similar things happen in many countries) and those who find a protest march exciting, and those who are only too ready to be violent and do a bit of light looting whilst they are at it, or set things on fire.
It doesn’t seem to occur to them that with so many in the crowds, the virus will have a field day.Maybe not with them,
Young and healthy but with their parents or anyone older they come in contact with.