Oppression of the People.
Its probably worth saying that prior to the tear gassing the protestors were being really overdramatic. The police were obviously trying to prevent the rally from moving down that side street. Is it really necessary to call them fascists?
We had protest rallies in San Francisco every day, (you and I even participated in some) and they never turned into the kind of conflict that seems to break out every time the hard core leftists are out in force. To think that police in different cities all over the world are specifically singling out these people for brutal repression but allowing all kinds of other protests of every variety to take place every day without similar issues is tin-foil-hat kind of thinking.
These protestors know that conflicts with the police will raise the profile of their demonstrations and they know that these conflicts reenforce their groupthink that the fascist state and the police are out to get them.
Newsflash: This isn't Egypt. You're not living in an autocratic state.
The fact is that these protestors are jerks. The police are not trying to prevent their voices from being heard, they are trying to manage the crowd and keep some of the streets passable. These protestors have no more right to the streets than other citizens who aren't participating in their struggle or don't agree with their views. It was not necessary or appropriate for the police to tear gas them, but you can be a victim of police brutality and an asshole at the same time.