I would like to point out that I think at least part of the story is missing here. I can't believe cops would randomly decide to beat people for no reason. I still think it's an abuse of power though.

Did the protesters have a permit? I think there's a rule, law and/or regulation saying you need to get a permit for a protest march so that the police knows what's going on and don't have to put in massive resources on a false alarm. Also, it enables them to redirect traffic and set up a perimiter between protesters, innocent bystanders and possible hooligans trying to interrupt the exercise of freedom of speech taking place there.
I'm not saying they didn't go through the necessary channels to get their protest approved, I'm just saying that looking at this video without any backstory or context, it sure looks like the police had no idea what was taking place. And if that is the case, then I'm not at all sure I'd side with the protesters on this. If uninformed, the policemen have no way of knowing that this protest is any more peaceful than a big, bloody riot in the making.

EDIT: Seeing this video reminds me of an episode of Seinfeldt, where a really annoying broad kept heckling Jerry when he did his standup routine.

Protest permit approval requirements violate the first amendment blatantly.

It's just a bunch of people walking down the street nonviolently, holding some signs, and shouting. That's no security threat. The worst the protesters did on this video was jay walk. Yeah there are cuts, but why would the video creator cut out infractions by the protesters? And the club is only for struggles in cases of resisting arrest. I see no evidence of resisting arrest.

Reactionist bullshit.

Maybe the police did get out of hand but the camera man doesn't even attempt to show the details and every second of him talking makes me want to punch him.

Looks like a bunch of mental defectives storming the UN, assaulting cops with flags. Sorry?

Do what police officers tell you.

I'm not saying the protesters were anything but peaceful, but what I am saying is that without any warning to the police, how are they supposed to know it's a peaceful protest? It's not like they're mindreaders.
Getting a permit for a protest isn't just about getting permition to voice your freedom of speech, a permit as I see it is first and foremost for making sure this doesn't happen, to make sure the police knows it's a peaceful protest and can in advance prepare to let it continue as best as possible. The same holds true for getting a bonfire permit, a mass rally permit, an inner city race permit and just about anything else that will cause havoc if unannounced. However, if the protesters did intend on turning their march into some kind of stunt and set up the police for bad publicity, this would surely be the way to do it.
I think we're forgetting that the police officers in this video are ordinary people doing their job just like us, and to suddenly have a huge mob of people causing havoc will surely stress someone out beyond belief. Then if the mob didn't intend on causing havoc really doesn't enter into it if you're the police officer in charge of maintaining order in that district, with no prior notification you'd be damn hard pressed to get things under control with no time to go and get information as to the nature of the approaching mob.
What I saw in this video concerning the policemen's behaviour is overreacting, yes, but surely not any real abuse of power. With the risk of sounding like a pessimistic jerk, how sheltered must we be to see this as police brutality? The harbour policeman is one thing, this is not the same in any stretch.
They lost control of the situation, and the taunting and heckling barrage from the protesters sure didn't help. The police officer making the verbal threats did apologize apparently as honestly as possible when the situation was starting to get under control, which is yet another reason why I'd be inclined to side with the police in this matter.
If indeed they didn't get a permit, it's incredibly irresponsible to first set up and execute such a large protest without mentioning it to the authorities, as it not only would interrupt the flow of the day for the city populace, but also intentionally make things harder for the authorities to do their jobs.

The protest doesn't just have to be announced, it has to be approved. Meaning, whoever is in charge of approval can easily silence speech. If all the protesters had to do was announce their intent to have a peaceful protest in advance, that would be OK by the first amendment. But that is NOT what the law says.

^so you go outside and break the law anyway?

I don't think that helps the situation really.

They're frustrated and rightfully so, but why come bother civilians that had nothing to do with your suffering?

They're just citizens. Take it out on the politicians damn it.

In fact, we strongly encourage that behavior.