I can understand the police's reluctance to step in. They may be accused by some of being used as establishment puppets to silence dissent.
I think it is perfectly acceptable to protest, but the behaviour of these men was not only extremely boorish but, in my view, could easily be perceived as threatening.
Several large men surrounding a lone woman - or man - and shouting in her/his face would certainly feel very threatening to me. Threatening behaviour is, as I understand it, unlawful, but it might be a bit tricky to nail down.