In the USA we have laws against advocating the violent overthrow of the government, and against general incitement to violence. But as long as they stay away from that line, all groups (even those that most of us find morally repugnant like neo-Nazis and the KKK) have a right to exist, proselytize, and peacefully assemble just like anyone else.
http://history.stackexchange.com/questions/2401/why-did-the-us-britain-france-denmark-and-sweden-vote-against-the-un-resoluti