The park was crowded, but the NPR caller was being deceptive. There's been one of those impromptu "memorials" in Union Square since the night after the attacks on the WTC, and it's grown daily (it has also been co-opted by antiwar sentiments to the point where it sickens me). That memorial attracts many visitors, as do the shops around the square.
STILL, even with all that going on yesterday, the crowd in the square was in the hundreds, not the thousands, and they were not all there for the peace rally! The peaceniks marched around the square singing at one point, and my estimate of their number was between 300-400. 12,000 would have been a sea of people that would have closed traffic for their march to Bryant Park or times Square or wherever they ended up. That didn't happen--it looked they all fit on the sidewalk!