BAGHDAD, Iraq — Assailants struck Shiite worshippers in three Iraqi cities today, killing at least 36 people in bombings and ambushes during the climax of ceremonies marking Ashoura, the holiest day in the Shiite calendar.

Police, meanwhile, questioned hundreds of suspects rounded up after a fierce weekend battle aimed at preventing even deadlier attacks.

The surge in violence came a day after Iraq's army announced it had killed the leader of a heavily armed cult of messianic Shiites called "the Soldiers of Heaven" in a fierce gunbattle aimed at foiling a plot to attack leading Shiite clerics and pilgrims in the southern city of Najaf.

The deadliest attack today occurred when a suicide bomber blew himself up among a crowd of worshippers entering a Shiite mosque, killing 16 people and wounding 57 in Mandalin, a predominantly Shiite city northeast of Baghdad and near the Iranian border.

To the north, a bomb left in a garbage can exploded about an hour earlier as scores of mostly Shiite Kurds were performing rituals commemorating the Islamic sect's holiest day in the Kurdish city of Khanaqin, also near the Iranian border.

At least 13 people were killed and 39 were wounded in that attack, police Maj. Idriss Mohammed said, adding that most of the victims were Shiite Kurds, who comprise the majority in the city, about 90 miles northeast of Baghdad. Most Kurds are Sunni but a minority are Shiite.

The two bombings occurred on the edge of the volatile Diyala province, where fighting has raged for weeks between Sunni insurgents, Shiite militiamen and U.S.-Iraqi troops.

Gunmen in two cars also opened fire on a bus carrying Shiite pilgrims to the capital's most important Shiite mosque at about 10:30 a.m. today in Baghdad, killing at least seven people and wounding seven others, police said.

The attacks came as millions of Shiites in Iraq commemorated Ashoura, marching in processions and beating themselves bloody in a frenzied show of grief over the 7th-century death of Imam Hussein, grandson of the Prophet Muhammad and one of the most revered Shiite saints.

Imam Hussein died in the battle of Karbala in A.D. 680. The battle cemented a schism in Islam between Shiites and Sunnis over leadership of the faith, a division that is at the heart of the sectarian violence that has spiraled in Iraq since the U.S.-led invasion in 2003, in particular since the Feb. 22 bombing of a Shiite mosque in Samarra.