A grizzly bear has you in his sights — not because you came between a mother and her cubs, but just because you're in the wrong place at the wrong time. So, what are you going to do?
If you try to run, you're dead meat. You can't outrun a grizzly, and the very act of running will trigger an attack.
Here's a link to a short video that shows exactly what to do. Note that grizzly bears often "bluff charge" just to scare off whatever is irritating them, without following through with a full-blown attack. If you run, it will chase you down and probably maul you — or worse. If you stand your ground and do not "threaten" the bear, it might do what this bear in the video did — go look for a tree to scratch its back.
Anyway, enjoy the video (there's a short commercial before the video starts, so be patient), and hopefully learn from it.