This video of a police officer punching a young woman is generating a lot of discussion around the web:
Unlike most viral videos of police using force against a suspect, this one seems to be generating a considerable amount of sympathy for the officer involved. Even on sites like Digg.com with a history of exposing and condemning police brutality, a majority of commenters feel that the officer acted more or less appropriately under the circumstances.
Since we can't see what took place prior to the video footage, it's impossible to completely critique the officer's handling of the situation. But it's clear that the woman who was struck had shoved the officer in an attempt to help her friend resist arrest. Though hard to watch, I doubt the officer's actions were technically illegal under the circumstances. He'd been assaulted, and police are authorized to use force when that happens.
I think I agree with Radley Balko that the officer should never have let a jaywalking arrest escalate to this point, but there's a reasonable counter-argument that police shouldn’t let uncooperative suspects off the hook simply to avoid a conflict. Flex Your Rights teaches passive non-cooperation such as remaining silent and refusing to consent to searches, and we strongly discourage physically resisting police during a detention or arrest.
The punch was ugly and unnecessary, but the behavior of the suspects here was also completely out of control. Even the most unreasonable arrest is better resolved by pleading your case to the judge than wrestling the officer on the street. You can't win that way, and there's no limit to how badly you could be hurt.