It was a realistic action thriller-y thing, and the plot was approximately as follows:

A violent gang (of teenagers, possibly) came to a quiet town and terrorized it/people. The hero of the movie was a man who had to sneak around and beat them. I remember it being a little like Die Hard, sneaking around in secret, and then fighting the bad guys eye to eye towards the end?
I think the gang had long coats, or possibly one member did, and that it was night most/all of the time, and I think the lead man - the hero - had a white t-shirt at some point/all throughout the movie.

VERY vague, but... anything might help. So I'll give it to you. But sadly that's all I've got. I could possibly recognize the male lead if I saw a picture! I can visualize him quite clearly, at least.
Thanks in advance!

What did the male leader look like?

Err- hard to say, my memory could be faulty. If you mean the male lead role of the movie, because I don't really remember anyone else at all.
He had short hair and was average built, or possibly/probably fit. But he wasn't overweight or super skinny, at least. He was in his thirties. Then again I was very young, and perhaps he simply seemed old in comparison. For all I know he was 20, and I thought he was super grown up or something.

I really don't have much to go on, at all. It's depressing.