At least three people have been killed and 20 others have been wounded in a bomb explosion in the southern Iraqi city of Karbala, security officials say. The explosion occurred in a parking lot in Oun area, north of the city, near a Shia shrine on Friday. The bomb was reportedly hidden under a parked car. Maj. Gen. Othman al-Ghanimy, commander of Karbala military operations, said the blast ignited five nearby cars, causing a second explosion when a gas tank caught fire. The explosion comes as thousands of Shia pilgrims are heading to Karbala ahead of the celebrations marking the birth anniversary of the 12th Imam, Mahdi (PBUH). Also on Friday, a roadside bomb targeting a police patrol killed one passer-by and wounded three others when it exploded in Baghdad's Doura district, an Interior Ministry source said. Various forms of violence have become a near daily occurrence in Iraq since the 2003 US-led invasion of the country. According to a study conducted by the prestigious British polling group, Opinion Research Business (ORB), the invasion led to 'violent deaths' of over a million Iraqi civilians. According to Iraqi government, 271 Iraqis have been killed in attacks last month, among them 155 civilians, 77 policemen and 39 soldiers.