Millions of Iranians, beating themselves with fists and chains, marked the climax of Ashura on Thursday, a day after a suicide bomber killed 33 people taking part in a procession for Shiite Islam's most revered mourning ritual.
Men, women and children dressed in black gathered in cities across Iran as the 10 days of rituals to mourn the death of Imam Hussein, the faith's third imam, reached their peak.
A grandson of the Prophet Mohammed, Imam Hussein, was killed by armies of the caliph Yazid in 680 AD. Tradition holds that he was decapitated and his body mutilated in the battle at Karbala, now in Iraq.
Iranian state television carried live footage of the Ashura processions in the capital Tehran, the second city of Mashhad, the Shiite clerical centre of Qom and the smaller cities of Yazd, Bam and Ardebil.
President Mahmoud Ahmadinejad attended a gathering in south Tehran.
The commemorations came a day after a suicide bomber from Sunni militant group Jundallah killed 33 faithful taking part in an Ashura procession in the southeastern city of Chabahar.
The group says it is fighting for the rights of the region's Sunni ethnic Baluchi community against the Shiite regime.
Last year's Ashura processions were overshadowed by the killing in Tehran of eight opposition demonstrators trying to keep alive street protests against disputed official election results which gave Ahmadinejad a new term that June.
This year there were no protest calls made by opposition leaders and their supporters were unseen on the streets of Tehran, which was the epicenter of last year's anti-government demonstrations.
As a result, security forces were able to focus on overseeing the Ashura processions, unlike in 2009 when they were joined by Basij militiamen in putting down the opposition protests, which have since completely petered out.
Shiites make up around 15 percent of Muslims worldwide. They represent the majority populations in Iran, Iraq and Bahrain and form significant communities in Afghanistan, Lebanon, Pakistan and Saudi Arabia.
Tight security in Iraq
In Iraq, authorities arrested 80 al-Qaeda-linked militants a general said were plotting attacks during Ashura ceremonies in Karbala, where the streets were crowded with 1.5 million pilgrims Thursday.
The arrests underscored fears of violence during the 10-day rituals in the shrine city that culminate on Friday, in a key test for Iraq's security forces ahead of a complete withdrawal of American troops in a year.
Vehicle traffic inside the city, 100 kilometers (60 miles) south of Baghdad, was brought to a virtual standstill as pilgrims were made to walk to the shrines of the revered Imams Hussein and Abbas, while all visitors were searched at checkpoints.
"Based on intelligence, soldiers were able to raid 14 terrorist cells and arrest 80 militants," General Othman al-Ghanimi said at a news conference Wednesday evening, noting that they belonged to a group called the "Boys of Heaven", an al-Qaeda-linked group.
"They were planning to attack pilgrims on Friday," added Ghanimi, the Iraqi army commander responsible for security in five provinces including Karbala.
Provincial governor Amal al-Din al-Har said that 1.5 million pilgrims had already entered Karbala for the commemorations. He added that a total of two million were expected by Friday, including 100,000 foreign visitors.
Around 28,000 soldiers and police are currently securing Karbala, with a further 7,000 available if needed.