It's apparently going to be pretty big (huge, even), with preliminary reports of 200,000 spectators expected as a bare minimum.
As for the acts, I know that Polish pop sensation, Ewelina Flinta will make an appearance at some point. Scientology enthusiast, Juliette Lewis, is also going to perform. As for the rest, a bunch of bands and musicians I've never heard of. They might have small reputations and followings, but no one on the A-list (what else do you expect from a free concert?)