Video was just posted on Twitter with a plea to spread the footage. It shows Philadelphia Police corner a large crowd between the police line and a steep hill. Then they bombard the crowd with tear gas even after the cloud is so think nobody can see and because they are trapped they can't go anywhere. This is nothing but excessive and disgusting. It is like shooting fish in a barrel while ordering them to leave the barrel.
The protesters of course are out protesting police brutality only to find themselves the victims of brutality themselves. These protesters are not hurting anyone. This could be the result of a systemic problem in which police are trained to keep using force until an order is complied with. For instance, if they trap someone, tell them to move, and they can't move, officers often apply force and say they are doing it to make them comply even though compliance is impossible. Kind of like when they beat people up and keep yelling "stop resisting."
UPDATE: The mainstream press are reporting that this happened on Interstate 676. The source link above this article has been changed to a link to a better video. The police also fired rubber bullets. Also, it turns out that there is a fence on the far right that is trapping them on that hill.