Baghdad, May 14 (IANS): Iraqi security forces responded to a sectarian riot overnight at a Sunni district in Baghdad as Shia pilgrims observed a major ritual at a holy shrine, an interior ministry source said on Thursday.

The disturbance occurred late Wednesday when dozens of young men among the pilgrims attacked and set fire to the building of the investment department of the Sunni endowment office, in Baghdad's northern district of Adhamiyah, the source told Xinhua news agency.

The office is responsible for running Sunni mosques across Iraq.

Every year, Shia pilgrims from Iraq and other Muslim countries walk through Baghdad's streets, including the Sunni district of Adhamiyah, heading toward the holy shrine district of Kadhmiyah, to observe the commemoration of the death of Imam Musa al-Kadhim, the 7th of the 12 most sacred Shia Imams.

Crowds also set fire to some houses and many cars nearby before security forces intervened and arrested several suspected rioters, the source said, adding that 15 people were wounded in the chaos.

Across the capital, authorities deployed more than 75,000 security personnel, manning dozens of checkpoints and patrolling the worshippers' routes, with army helicopters hovering overhead.

A traffic ban was imposed inside and around the district of Kadhmiyah, and troops blocked many roads leading to the main pilgrim routes across the capital, in addition to blocking all entrances to the holy site.

Only pedestrians were allowed but not before crossing several rings of security checkpoints.

The office of Prime Minister Haider al-Abadi, who visited Adhamiyah and Kadhmiyah Thursday morning in the wake of the rioting, attributed the overnight chaos to "some infiltrators among the pilgrims" who were agitated by a rumour that terrorists wearing explosive belts attempted to attack the pilgrims.