I kind of have to agree with Baraka_Guru on this. Unless or until we have more information, it's impossible to say whether the police action was justified.
We don't see any of the events that preceded the arrival of the police force on the scene, for one. I don't know how they do things in Pittsburgh, but it's my assumption that if police are arriving in riot gear it's because they have concerns that the situation may escalate.
A quick Google search turns up an MSNBC article detailing the events of Thursday, Sept 24 in Pittsburgh -- according to them, over $50 000 in property damage was caused by rioters, and four unnamed individuals were charged with aggravated assault.
Boisterous protesters at G-20 summit - Life- msnbc.com
I think there's more to this story than is available in either the article or video. Given that, I really don't feel like I have enough information to comment on the propriety of the officers involved.