SimchaMinute by Rabbi Avigdor Miller
It was something to see: two million people were in Jerusalem for the Korban Pesach. The streets were jammed. There was no place even to sit.
As they ate the Korban Pesach, they were singing Hallel. The gemara says the Hallel was so powerful that the roofs of Yerushalayim were swaying up and down, almost caving in from the noise. It was a tremendous experience. — The Wedding Simcha, or Rebuilding the Ruins of Yerushalayim (#E-147)