BAGHDAD (AP) — Tens of thousands of Shiite pilgrims converged Tuesday on a golden-domed Shiite shrine in Baghdad to commemorate the anniversary of the death of a revered imam as authorities tightened security measures in the Iraqi capital amid concerns of more attacks by Sunni extremists.

Security forces blocked all major roads in Baghdad to protect the pilgrims, who have been making their way on foot this week to the shrine in the northern neighborhood of Kadhimiyah, where the 8th century Imam Moussa al-Kadhim is buried.

However, Sunni extremists, who see Shiite as apostates, have still managed to launch several attacks on the pilgrims, including a car bombing on Monday that killed 18 pilgrims and wounded 45 people. Shortly after that explosion, the Islamic State group claimed responsibility for the attack in an online statement.

On Tuesday, when the annual commemoration events culminate, some of the pilgrims in Kadhimiyah pounded their chests in grief while others flagellated themselves with chains or carried symbolic coffins of the imam, wrapped in a green cloth.

Inside the shrine, clerics recited the story of how al-Kadhim, a great-grandson of the Prophet Muhammad and one of the 12 holiest Shiite imams, was poisoned by the Abbasid caliph, Haroun al-Rasheed.

The annual Shiite pilgrimage has brought some ease to the country's simmering political crisis.

On Sunday, anti-government protesters decided to disband their demonstration — at least temporarily — in Baghdad's heavily fortified Green Zone that they had stormed a day earlier. Followers of influential Shiite cleric, Muqtada al-Sadr have been launching protests and sit-ins to demand for reforms and new government.