At least three pilgrims were killed in a suicide bomb explosion in Adamiyah district of northern Baghdad on Wednesday, local police source told Xinhua. A suicide bomber blew himself up among the pilgrims who were marching on foot in Baghdad, where security has been tightened ahead of a major Shiite ritual, killing at least three, the source told Xinhua on condition of anonymity. The attack came a day after a series of mortar and bomb attacks that killed up to six Shiite pilgrims and wounded some 40 others as thousands of them were travelling on foot across Baghdad streets heading to a holy shrine in Kadhmiyah to observe the major Shiite ritual whose climax will be on Thursday. About 1 million pilgrims from many parts of Baghdad and other provinces are expected to gather near the Imam Mussa al-Kadhim shrine in Baghdad's northern district of Kadhmiyah for the annual commemoration of the death of the seventh of the most sacred 12 Shiite Imams. 