The sectarian violence, which has decreased since the peak in 2006 and 2007 and was expected to be even worse on Sunday, is typical during Ashura, which commemorates the killing of Imam Hussein, the grandson of the founder of Islam. This year, Ashura and its week and a half of ceremonial events coincided with Christmas, stoking fears of additional targets.

Four people were killed and 28 injured Sunday morning when a roadside bomb exploded near a crowd of Shiite worshipers in the northern city of Tuz Khormato, an ethnically mixed city of Kurds, Turkmen and Arabs. It is in Salah ad-Din province, less than 50 miles south of Kirkuk, considered part of the disputed territories left over from Saddam Hussein's ethnic-cleansing policies.

Reports had surfaced that the Iraqi minister of youth and sports, Jassim Mohammed Jafar, was targeted in the blast and his bodyguard killed. Mr. Jafar told the Aswat al-Iraq news agency that his convoy left the scene before the blast.

In Baghdad, a bomb exploded inside a minibus in the Mansour district, killing one person and injuring four others, according to sources in the Ministry of Interior.

There was no violence reported in Karbala, in southern Iraq, one of the holiest Shiite cities, where Imam Hussein was killed and buried. Hundreds of thousands of pilgrims flocked to Karbala for the pinnacle of the Ashura commemoration Sunday, which saw an influx of between 25,000 and 50,000 additional Iraqi police officers, according to Interior Ministry sources.

Karbala is the scene of the largest commemorations in Iraq, which include self-flagellation with fists, chains and swords.

Violence that mostly targeted Ashura pilgrims left at least 40 people dead and dozens more injured from Wednesday through Saturday throughout Iraq. Christians also clashed with Shiites north of Mosul Thursday night, but no one was killed.

One person was killed Wednesday in an attack outside a church in Mosul.