Iraqi officials say police have deployed tens of thousands of troops to prevent attacks on Shiites observing an annual pilgrimage to Karbala, sending helicopters to hover over the holy city to look out for insurgents.

Bombings during another Shiite pilgrimage last month killed more than 100 people. The attacks have not stopped throngs of white-robed faithful from gathering Friday to mark the birth of the ninth-century Shiite leader known as the Hidden Imam.

Vehicles have been banned inside Karbala, south of Baghdad. Provincial councilman Hussein Shadhan al-Aboudi says more than 40,000 police and soldiers are on the streets and 14 helicopters are monitoring the crowds.

Iraq's al-Qaida-linked Sunni insurgents often attack Shiite pilgrims and other targets to destabilize the government and stoke sectarian tensions six months after U.S. troops withdrew.