At least three people were killed and 17 others wounded in four car bomb attacks in Iraq's holy Shia city of Karbala, a police source said.Two booby-trapped cars exploded at Bab Twuirij district in northern Karbala, some 110 km south of Iraq's capital Baghdad, and two others detonated in the central and eastern parts of the city, the source told Xinhua on condition of anonymity.The toll could rise as ambulances and police vehicles were evacuating the casualties to hospitals, the source said.Earlier in the day, a suicide bomber blew up a Shia mosque in central Baghdad, leaving 10 people dead and 31 wounded.A series of deadly attacks over the past few days that targeted the predominantly Shia districts in Baghdad and some other Iraqi cities left dozens dead and wounded. Iraq has been witnessing some of the worst violence in years. Terrorism and violence have killed 5,576 civilians in the first half of the year, and wounded 11,666 more, according to a recent UN report.