(Xinhua) -- A group of Shiite pilgrims enroute to attend an major ceremony were attacked by unidentified gunmen in northern Baghdad Friday night, leaving seven people killed, an Interior Ministry source told Xinhua. The attack occurred at about 9:30 p.m. in al-Adal district while the pilgrims were walking to the Imam Mousa al-Kadim shrine in Kazimiyah district for a ceremony. Thousands of Shiite pilgrims from many parts of Baghdad and other provinces are gathering near the revered shrine in Kazimiyah district for the annual commemoration of the death of the seventh of the 12 most sacred Shiite imams. In last year's ceremony, about 1,000 Shiite pilgrims were killed in a deadly stampede on a bridge over the Tigris River, which was triggered by a fake suicide bomb alarm. The Iraqi authorities have tightened security in the capital and imposed a two-day vehicle ban in some neighborhoods starting from 9 p.m. (1700 GMT) on Friday to 6 a.m. (0200 GMT) on Monday.