Karbala will see the climax of the Shia Muslim festival of Ashura on Tuesday, with pilgrims marking it freely for the first time in 30 years.
The festival often turned into a political rally
Hundreds of thousands are in the Iraqi holy city for events to mourn the death there of Imam Hussain, grandson of the Prophet Mohammed, in the year 680.
His death is being marked by rhythmic chanting and ritual self-flagellation from devotees clad in black.
Ashura was severely restricted under former Iraqi leader Saddam Hussein.
The BBC's Heba Saleh says that he feared the mass festival's potential to foment rebellion.
Many of the devotees in Karbala this year have come illegally from neighbouring Iran, which, like Iraq, has a majority Shia population.
A lot of the new arrivals have been sleeping in tents or on the streets with volunteers providing them with food.
Security is tight, with checkpoints set up around Karbala and religious militias guarding holy shrines.
Years of repression
The death of Hussein at the hands of the Caliph Yazid cemented the split between Sunni and Shia Islam over who should lead the faith.
Shia were repressed under Saddam's Sunni-dominated regime, toppled last year by a US-led coalition which now occupies the country.
Observation of Ashura was hampered by a ban on the practice of ritual self-beating and cutting the skin.
Pilgrims from Iran - a wartime enemy of Saddam Hussein's Iraq - were not welcome at Karbala while he was in power, one of Shia Islam's holiest sites.
Our correspondent notes that other rulers before Saddam Hussein similarly sought to curb the mass Shia rituals, from the Ottomans and Mamelukes to the British in the last century.
The climax of Ashura comes a day after Iraq's interim Governing Council agreed a draft constitution for the country.
One mourner, Dr Hussein Ashestani, said political protest was a central part of this year's Ashura.
"A lot of people are coming here really to express their determination to practice their political role and express their voice very loudly," Dr Ashestani told the BBC.
"They are all insisting that we will not accept anything but elections and will not recognise any unelected body."
An AFP correspondent noted that a group of about 150 students from
Baghdad University were chanting anti-coalition slogans such as "Down, Down America" and burning an American flag.
This year's event coincides with the growing dominance of Shia in post-Saddam Iraq - leading to fears that disgruntled Sunni militants may target the celebrations.
To prevent this, Karbala has been ringed by security forces - with Polish soldiers policing the town's entry points and Shia militia guarding its streets and shrines.