This showed up on both GowanusLounge.com and Gothamist today. I'm pretty sure it's 4th Avenue between 60th and 61st, but none of the signs look clear...however, the construction fencing looks like what's on the 4th Ave and 61st corner. Check it out if you're willing to be harangued with shaky camera work and salty language.
The summary is that a man whom bystanders believe has just robbed and then cut someone with a razor blade is apprehended by those bystanders. He's on the ground, surrounded by men (and one very angry woman) scolding him and protecting him from people who want to beat him up. They are waiting for the police to show up. The police finally get there (after one man gets a kick in at the kidney) and cuff and search him.
A caveat anytime something like this is shown: It is dangerous, albeit commendable, to chase and catch a thief (vandal, mugger, generally naughty person) because you never know what they might have up their sleeve. Also, situations like this can quickly become even more ugly if those involved end up taking not just the capture but also the punishment into their own hands. The men involved here did a good job of keeping physical retribution to an absolute minimum (from what we see on the video), although the epithets dashed at the man on the street (literally) are pretty ugly.
Here's the link from GowanusLounge.