Arbaeen marks the 40th day after the martyrdom anniversary of Imam Hussein, the Prophet Muhammad's grandson and the third holiest figure in Shia Islam.
The Iraqi defense and interior ministries have deployed 35,000 security forces to secure roads for the pilgrims.
The memorial ceremony constitutes the first challenge for the Iraqi forces after the withdrawal of US troops from the country.
Despite the recent spate of terrorist attacks which targeted pilgrims in different parts of Baghdad, many pilgrims kept walking from their cities to Karbala.
Karbala's military operations center announced that it has new plans to provide security on Arbaeen.
The local government of Karbala said that security preparations have been made since ten days ago to prevent new terrorist attacks against the pilgrims.
The third Shia Imam was martyred along with 72 of his companions while fighting for justice during the battle of Karbala over 1,300 years ago.