Boo Birds Dump All Over Pam Anderson

7/31/2008 4:00 PM PDT BY TMZ STAFF
At first, pappers and fans played by the rules and waited excitedly -- behind a barricade -- for Pam Anderson to come out of Letterman. But when she refused to give it up -- twice -- a sea of boos quickly flooded in.

Only after the hisses did she come back out and the scene became a paparazzi free-for-all.

Maybe they were just saying, "booooooooooooobs."