BAGHDAD — For the second time in two days a suicide bomber struck outside a Shiite mosque in restive Diyala province north of Baghdad on Thursday as worshippers prepared for one of the most important days in the Shiite calendar. Police and witnesses said at least 11 people were killed.

The bomber blew himself up at a civilian checkpoint erected to protect the Shafta mosque in Baquba during the ceremony of Ashura, which commemorates the 7th century slaying of Imam Hussein, the Prophet Muhammad's grandson and a revered figure to Shiites around the world.

Witnesses said that moments before the blast one policeman, named Baseem, seized the bomber and began shouting: "stranger, stranger," but was too late to stop the explosion, which killed both men and injured 15 others, six of them critically.

On Wednesday, a woman suicide bomber killed eight people near another Shiite mosque in Diyala province about 10 miles south of Baquba.

The Baquba mosque, which is in what many considered a safe area of the city, was the site of a suicide attack three months ago during a reconciliation conference when 15 people were killed, said Mohammed al-Zubaidie, who runs the mosque.

It was surrounded by the distinctive black banners being readied for Shiites to carry to the southern city of Karbala for the annual pilgrimage commemorating the killing and decapitation of Hussein in Karbala by armies of the Sunni caliph Yazid during a power struggle.

As the Shiite-led government imposed a two day vehicle curfew in Baghdad, Diyala and southern provinces to protect the hundreds of thousands of pilgrims walking to Karbala in the coming days, survivors of the Baquba attack said they would not be deterred.