There have been at least 965 confirmed deaths. The incident happened on a river bridge as about a million Shias marched to a shrine for a religious festival. The bridge links the staunchly Sunni area of Adhamiya on the east bank of the Tigris and the Shia area of Kadhimiya on the west bank.
BBC Middle East analyst Roger Hardy says that, because of radical Sunni attacks on big Shia gatherings in the past, it was not unreasonable for the worshippers to be nervous.
Earlier, mortar rounds had been fired into the crowd, killing at least seven people. About 36 others were injured when four mortar rounds landed close to the Kadhimiya mosque.
Iraqi defense Minister Saadun al-Dulaim said "People swarmed the bridge. There had to be a search operation at the end of the bridge, so crowds gathered and a certain scream caused chaos ... And this sorrowful incident took place."
During the crush, iron railing on the bridge leading to the shrine gave way and hundreds of people fell into the water.
It seems to me that few people in the Western world know the difference between the Sunnis and the Shia, and that knowing the difference would go a long way in helping us to understand middle eastern conflicts. Much like knowing the difference between Protestants and Catholics helps one to understand the fighting that has gone on in Ireland.
This event could be compared to shouting fire in a theatre. People in North America have been killed, crushed to death in stampedes many times. There was a stampede in a club during a concert around two years ago when fireworks lit part of the stage on fire. Officials have had to ban rush seating in stadiums after various deaths over the years. In fact many universities have coursed that study how to design things that will help prevent crowds from crushing their own weaker members. I have seen computer simulations that show how people leave rooms and airplanes with different configurations.
The main difference is that we generally don't gather in groups of a million or more. We don't have to search every single person who wants to cross the bridge when we do have a group that big, and generally people don't shout fire in our theatres.