later it was said it was a publicity stunt in which the police and the crowd knew about.they rehearsed it before hand.its still funny though.
http://www.youtube.com/watch?v=GtrBZJ9pYC0
heres them practicing it-
Take 1
http://www.youtube.com/watch?v=TwW1XUpqmNw&NR
Take 2
http://www.youtube.com/watch?v=OyoiJpOcW6g