Officials said some 15 million pilgrims will have passed through Karbala by the end of the commemorations, braving the threat of deadly violence targeting the Shiite community that has killed dozens in recent weeks.
Arbaeen marks 40 days after the Ashura anniversary commemorating the slaying of Imam Hussein, one of Shiite Islam's most revered figures, by the armies of the caliph Yazid in 680 AD.
In recent days, waves of mourners have overflowed Hussein's shrine in Karbala, demonstrating their guilt and remorse for not defending him by beating their heads and chests in rituals of self-flagellation.
"I have been walking for 12 days," said Adil Salim, a devotee from Iraq's southern port city of Basra. "Despite the threats and the exhaustion, we insist on taking part in these commemorations."
"We will never stop, no matter what the terrorists do."
Sad songs blared from loudspeakers throughout the city and black flags fluttered alongside pictures of Hussein and his half-brother Imam Abbas, both of whom are buried in the city.
Karbala provincial governor Amal al-Din al-Har said he expected that some 15 million pilgrims will have visited the shrine city in the two weeks running to Arbaeen's climax on Saturday.
Included in that figure, he said, were around 200,000 devotees from outside the country.
"All services are going smoothly," he told AFP, but noted that the province's electricity and transportation networks were overwhelmed by the sheer number of pilgrims.
Some 35,000 police officers and soldiers were handling security around the city, 110 kilometres (70 miles) south of Baghdad, and eponymous province as Shiite pilgrims carried out their traditional walk to Karbala.
Iraq's security forces have also deployed 500 policewomen to assist in checkpoint searches, as well as sonar detectors and sniffer dogs, according to Lieutenant General Othman al-Ghanimi, who commands forces in five provinces across central Iraq, of which Karbala is one.
He said they have so far arrested 60 insurgents they alleged were planning attacks during the pilgrimage, and defused 14 roadside bombs.
While Karbala itself has not suffered any attacks during Arbaeen rituals, bomb attacks nationwide have targeted Iraq's majority Shiite community, with the deadliest assaults falling on January 5, when 70 people were killed in bomb blasts in Baghdad and south Iraq.
The seventh century battle near Karbala is at the heart of the historical division between Islam's Sunni and Shiite sects, a split that fuelled sectarian violence between Iraq's majority Shiite and smaller Sunni population since the 2003 US-led invasion.
Shiite pilgrims visiting Karbala are regularly targeted in attacks by Sunni extremists.
This year is the first time Iraqi troops have been solely charged with security for Arbaeen since the US-led invasion of 2003. American troops, who previously helped with surveillance and reconnaissance, completed their withdrawal from Iraq last month.
Now-executed dictator Saddam Hussein's Sunni-dominated regime barred the vast majority of Ashura and Arbaeen commemorations.
Shiites make up around 15 percent of Muslims worldwide. They represent the majority populations in Iraq, Iran and Bahrain and form significant communities in Afghanistan, Lebanon, Pakistan, India and Saudi Arabia.