The pilgrimage marks the 40th and final day of mourning for Imam Hussein, the Prophet Muhammad's grandson. The commemoration will take place on March 20.
Police say a mortar shell exploded in the center of the city today, adding that there were no casualties.
In and around Karbala, thousands of Iraqi police have been deployed for fear of possible sectarian violence.
Army troops have also been posted in the desert outside of the city to prevent possible attacks by Sunni Muslim groups.
Two years ago, coordinated attacks left 181 people killed in Karbala.
The Golden Mosque before the 22 February bombing (courtesy photo)
UNDER THE GOLDEN DOME: The Iraqi city of SAMARRA is the site of two major Shi'ite shrines. Consecrated in 852, the Golden Mosque is said to hold the remains of two Shi'ite imams: Ali al-Naqi and his son, Hasan al-Askari. A second shrine marks the place where the hidden -- or 12th -- imam, al-Mahdi, son of Hasan, went into hiding.
Imam Ali and Hasan were imprisoned in Samarra, the capital of the Abbasid Dynasty, by Al-Mutawakkil Ala Allah Jafar bin al-Mu'tasim (821-861), who is considered the last great Abassid caliph.
According to historical accounts, al-Mutawakkil felt threatened by the growing influence of Shi'ite Islam and Imam al-Naqi, who was based in Medina. Al-Mutawakkil thus brought Imam Ali and Hasan to Samarra in 848 and imprisoned them inside a military fort. Henceforth they became known as al-Askari (military) because of the location of their imprisonment.
Following al-Mutawakkil's death in 861, his successor had Imam Ali poisoned in 868. Hasan died in 874.
Imam Ali al-Naqi -- the 10th Shi'ite imam, commonly referred to as Imam Ali al-Hadi -- and his son, Hasan al-Askari, the 11th imam, are buried under the Golden Dome, which was a gift from Persian ruler Nasr al-Din Shah (1848-96). The dome's construction was completed in 1905. Also buried in the shrine are Hakimah Khatun, the sister of Imam Ali, and Nargis Kahtun, Imam al-Mahdi's mother.
The second shrine in the complex marks the place where Shi'a believe Imam al-Mahdi (b. 868), the 12th and final imam, went into hiding. According to Shi'ite tradition, Imam al-Mahdi, the son of Hasan al-Askari descended into a cellar under the present-day shrine and disappeared. Shi'a believe that he never died, and he will return on Judgment Day.
MORE: For more information on Shi'ite and Sunni sectarianism in Iraq, see: