The worst crowd I faced was in 2001 in Florence, Italy. My friend and I went to the Duomo for the Scoppia della Carre ceremony, which happens every Easter morning. Our landlady had told us the best place to watch it was the square right between the Duomo and the Baptistery. By the time we got there, a huge crowd had already formed. It was hard to see, but the procession slowly came towards the Duomo, then waited outside the doors. The “cart” was actually more of an ornate structure – it was roundish, with a dome, and looked very old. At 11 am, the priest saying Mass inside the cathedral releases a mechanical dove, which flies out into the square and ignites the fireworks attached to the cart. So for the next 15-20 minutes, fireworks went off, along with huge puffs of smoke and noisy explosions. It was slightly alarming at times, since the fireworks were practically on top of the huge crowd. At one point, an ambulance barreled its way through the swarm of people, barely stopping while people rushed to get out of its way.
Very crowded, and somewhat scary between the fireworks and the ambulance plowing through, but still worth it. Our landlady also bought us an Easter cake that was one of the most wonderful things I've ever eaten. It was bread-like; very dense and sweet and flavored with bits of orange rind, and covered with a rich, crusty sugared topping.