Shi'ite pilgrims in their millions have flocked to Baghdad from within and outside Iraq to commemorate Musa al-Kazim (745-799), seventh of the 12 Imams revered by the Twelver Shi'ites. According to the Iraqi government, the pilgrims number as many as five million.
Specifically, the event marks the anniversary of the death of Musa, allegedly at the hands of the Abbasid caliph Harun al-Rashid.
The BBC has put together a photo essay on the festival; you have to see the pictures to appreciate the scale of this thing.
Only three people of the up to five million pilgrims were killed in communal violence, which is being hailed as a victory for the Iraqi army, which was providing security at the gathering.
[This post was written in Istanbul.]