Thousands of Shia Muslims head for Karbala as Baghdad is placed under curfew.
Security is especially tight at the Kadhimiyah mosque, where the imam is said to be buried, and has been the site of previous attacks.
Brigadier General Qassim al-Moussawi, Iraq’s military spokesman, said that 100,000 members of the Iraqi security forces, along with US reinforcements, had been deployed in Baghdad, while US-led forces will provide air support.
A vehicle ban also has been imposed in Kadhimiyah until the end of the festival on Tuesday, al-Moussawi said.
Security forces deployed for the event include a team of female guards to search women.
On August 31, 2005, at least 965 people died in a stampede at a Baghdad bridge near the area triggered by rumours that a suicide bomber was in their midst. The rumours had developed following a mortar attack on the mosque that killed seven people.
In other violence on Sunday, a bomb wounded a member of an Iraqi provincial council and killed two of his bodyguards in the city of Fallujah in what police said was an assassination attempt.
Zeki al-Mohammedi, a member of the al-Anbar provincial council and head of the Fallujah branch of the Iraqi Islamic Party, escaped with minor wounds when the bomb exploded inside the garage of his home.