A blast in northeastern Iraq killed five people Sunday in a crowd of pilgrims who had gathered for the most important Shiite religious observance of the year, authorities said.

It was the latest in a string of attacks targeting Shiites during the 10 days of religious ceremonies that reached their high point Sunday. The roadside bomb in the town of Tuz Khormato, 110 miles northeast of Baghdad, wounded 28 people, police and medical officials said, speaking on condition of anonymity because they were not authorized to release information.

Sunday's commemorations marked the climax of Ashoura, the yearly mourning period in which Shiite Muslims remember the seventh century death of the Prophet Muhammad's grandson, Imam Hussein, in a battle in the central city of Karbala.

Pilgrims traveling to the holy city and others preparing for commemorations elsewhere in Iraq over the past week have come under repeated attack by insurgents seeking to re-ignite sectarian violence that brought the country to the brink of civil war two years ago. Dozens have been killed and more than 150 injured.

Othman al-Ghanimi, the Iraqi provincial police chief in charge of Karbala, said he was expecting up to 3 million visitors, including foreign pilgrims from other Muslim countries, such as Iran, Bahrain and Kuwait.

Ashoura processions, when men beat themselves with swords and chains to demonstrate their grief, were banned under former dictator Saddam Hussein's Sunni-dominated regime. Publicly marking the holiday — despite the threat of attacks — has become a demonstration of strength for Iraq's majority Shiites.

The Iraqi government has flooded the area in and around Karbala with 25,000 extra security personnel. The government is eager to demonstrate that the withdrawal of American forces will not leave a security vacuum for the insurgents to exploit.

American forces provided explosives-detecting equipment and were offering air support, al-Ghanimi said. He added that the city was encircled by four rings of security. Al-Ghanimi said Iraqi authorities had already thwarted some attempted attacks against pilgrims.

"Our security efforts led to the arrest of four terrorist groups who were intending to launch terrorist attacks against pilgrims and also the arrest of a terrorist with an explosives belt on the Baghdad-Hillah road who was about to detonate himself among visitors," he said.

The road between the capital and the city of Hillah to the south is a main route for pilgrims.

But as security has been tightened in Karbala over the years, insurgents have expanded their targets, often attacking pilgrims as they travel to and from Karbala or in smaller processions that take place around the country.

Another security official said around 400 processions had begun moving toward Imam Hussein's shrine.

Television images and pictures of the men showed them using swords and chains to beat themselves. Some had blood spilling down their faces, staining their white robes a bright red.

The official spoke on condition of anonymity because he was not authorized to speak to the press.