By Khaled Farhan

NAJAF, Iraq, Oct 3 (Reuters) - Shi'ite pilgrims converged on the Iraqi city of Najaf on Wednesday amid tight security for their first major religious ceremony since clashes between Shi'ite militias killed 52 people in nearby Kerbala in August.

Thousands of pilgrims arrived for the event, due to begin at dusk, commemorating the killing more than 13 centuries ago of Imam Ali, Prophet Mohammad's highly revered son-in-law and first Imam of Shi'ite Islam.

Residents of Najaf, 160 km (100 miles) south of Baghdad, said tight security outside the city and fears of fresh violence meant that turnout was considerably lower than last year, when local officials said 3 million pilgrims attended.

Pilgrims go through three sets of checkpoints before boarding buses and minibuses with special permits that carry them into the city and drop them off 2 km (one mile) away from the shrine.

Iraqi army Major-General Uthman al-Ghanimi outlined security plans that aimed to prevent mortar and rocket attacks fired from outside the city as well as close air monitoring of groups on the ground.

"We have more than 30,000 security personnel in the city and its outskirts," Ghanimi said.

"The preparations are very good and we reassure our people in Najaf, the neighbouring provinces and those who arrive from the region to embrace this honourable occasion," he added.

Iraqi security forces are primarily responsible for the security operation, with U.S. troops a background presence.

The Kerbala fighting was believed to involve Iraq's two most powerful Shi'ite militias, the Badr Organisation linked to the Supreme Islamic Iraqi Council (SIIC) and Shi'ite cleric Moqtada al-Sadr's Mehdi Army.

Sadr ordered a six-month freeze of his militia's activities immediately after the clashes to calm rising tensions between Shi'ite communities.

A statement from the Mehdi Army this week urged Sadr followers making the journey to Najaf to refrain from carrying portraits of Sadr and avoid wearing their trademark shrouds in an effort to blend in and avoid trouble with security forces.

Imam Ali was fatally wounded in the 7th century while praying in the main mosque of nearby Kufa. He was the leader of the Islamic Caliphate at the time of his assassination.

Though he is also revered by the world's Sunni Muslims, his death triggered the theological and political split in Islam that resulted in the Sunni and Shi'ite sects.

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