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Hadi Mizban, Associated Press
In this photo taken Thursday, July 5, 2012, Shiite pilgrims light candles and fireworks inside the courtyard of the Imam Hussein shrine during the annual festival of Shabaniyah, which marks the anniversary of the birth of the ninth-century Shiite leader known as the Hidden Imam, in Karbala, 50 miles (80 kilometers) south of Baghdad, Iraq. Religious Shiites refer to Mohammed al-Mahdi as the "Hidden Imam," believing he was spared death and will return to Earth to bring peace and justice.

BAGHDAD — Thousands of police backed by helicopters patrolled Iraq's holy city of Karbala on Friday, assigned to prevent attacks against hundreds of thousands of Shiite Muslims observing an annual pilgrimage, officials said.

Bombings at another Shiite pilgrimage last month killed more than 100 people, part of a fierce and sustained terror campaign blamed on al-Qaida-linked insurgents seeking to destabilize the government and stoke sectarian tensions six months after U.S. troops withdrew.

The threat of violence did not stop the throngs of white-robed faithful, who crowded in and around Karbala's golden-domed main shrine to mark the birth of the ninth-century Shiite leader known as the Hidden Imam.

Vehicles have been banned inside Karbala, 80 kilometers (50 miles) south of Baghdad, since Tuesday, when two car bombs outside a market in the holy city killed five people and wounded 30 others.

More than 40,000 police and soldiers were patrolling the holy city and 14 helicopters were monitoring the crowds and looking out for insurgents, provincial councilman Hussein Shadhan al-Aboudi said. He estimated more than 1 million pilgrims had gathered for Friday's peak of the ceremonies.

"The terrorists have once again failed to deter the pilgrims from visiting the Shiite holy shrines," al-Aboudi said. "The terrorists have been attacking the pilgrims in the past, yet the number of pilgrims is increasing and their determination is getting stronger."

Iraq's al-Qaida-linked Sunni insurgents often attack Shiite pilgrims, along with government officials and security forces including Sunni militiamen working with the government.

One goal is to try to reignite the mass sectarian violence between Sunni and Shiite communities that brought Iraq to the brink of civil war starting six years ago.

Al-Aboudi, a Shiite, said that the recent wave of attacks would not provoke retaliation from Shiites.

"Everybody knows that sectarian strife would only serve the agenda of terrorists and the enemies of Iraq," he said.

While bloodshed in Iraq has dropped sharply since the 2006-2008 sectarian battles, terror attacks have remained a fact of life for ordinary Iraqis. The violence has intensified since June, with 300 people killed in attacks coming every few days.

The sustained level of attacks indicates the insurgents are emboldened by Iraq's protracted political crisis, which pits Shiite Prime Minister Nouri al-Maliki against Sunni, Kurd and rival Shiite politicians who say they are being sidelined.

Experts say the crisis in neighboring Syria may also be fanning the Iraqi insurgency, as some weapons intended for rebels fighting forces loyal to Syrian President Bashar Assad could be falling into the hands of Iraqi militants as they cross the country.

Iraq's Foreign Minister Hoshyar Zebari on Thursday said he had "solid information" that al-Qaida militants were also crossing from Iraq to Syria to carry out attacks, warning of a violent spillover that could shake the Middle East.