(Corrects typographical error in first paragraph)

By Khaled Farhan

NAJAF, Iraq, Oct 4 (Reuters) - The first major religious ceremony in Iraq since clashes between Shi'ite militias killed 52 people in August has passed without incident, Iraqi security officials said on Thursday.

Iraqi security forces estimated about 1.5 million pilgrims converged on the city of Najaf to commemorate the killing more than 13 centuries ago of Imam Ali, Prophet Mohammad's revered son-in-law and first Imam of Shi'ite Islam.

More than 30,000 security guards, aerial surveillance, numerous checkpoints and concrete barriers meant pilgrims were shepherded safely through the outskirts of Najaf before buses and minibuses ferried them closer to the shrine.

"There have been no breaches or incidents, the performance of the security forces has been very good," said Colonel Ali Nomas Jereo, spokesman for the security operation.

However, fear of violence meant the number of Shi'ite pilgrims who came to Najaf, 160 km (100 miles) south of Baghdad, was about half the turnout last year.

"We had a good visit. We were satisfied with the security," said Mohammad Naji, 27, a pilgrim from the capital Baghdad. "We found there was cooperation between the people and the security forces in the city, we did not feel fear or tired."

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

The fighting in August in the nearby city of Kerbala 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.

"I was surprised by all the barriers. I went through about 20 checkpoints before arriving in the city, but the governors of Najaf helped the pilgrims by providing transport inside the city," said Mohammad Jassim, 30, a pilgrim from Basra.

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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