Police said the attack on Monday occurred on the road leading from Hilla, 100km south of Baghdad, to Karbala, where this week pilgrims will mark Arbain, the end of an annual mourning ceremony.
Earlier, in Hilla, three policemen and two civilians were killed when a booby-trapped bicycle exploded next to a police car.
Police said someone managed to leave the bicycle next to the parked police car, which was guarding the highway leading from Baghdad to Hilla in al-Musayyib area.
Two other policemen and several civilians were also wounded in the attack.
In Baghdad, at least five people, including an Iraqi police officer, were killed and scores of others wounded in attacks throughout the city.
Senior Iraqi police officials have
been targeted in recent days
A colonel in the Iraqi police was shot dead while a roadside bomb targeting a police patrol left at least two people dead and at least five injured.
Colonel Abd al-Karim Fahid, chief of the Balat al-Shuhada station in the capital's southern Dura district, was killed along with his driver as he headed to work at about 8am (0500 GMT), an interim Defence Ministry source said.
Across the Tigris River that bisects Baghdad, a roadside bomb exploded near a police patrol in the Hay al-Amil area, killing one policeman and a road cleaner, and wounding five others.
A source at Yarmuk hospital said his facility received one dead policeman and seven wounded, including six policemen, from the attack.
In the northern city of Mosul, two Iraqi army soldiers were injured when attackers opened fire on their car, Dr Baha al-Din al-Bakry said. The two were dressed in civilian clothes at the time of the attack, he said.
A university professor, Waad Muhammad Husain, was also fatally shot as he was driving from home to work in the Zanjily neighbourhood, al-Bakry said.
Meanwhile, video footage posted on the internet by a group calling itself al-Qaida in the Land of Two Rivers, purportedly showed the execution of a man identifying himself as Colonel Riyad Gati Ulaiywi.
Groups have abducted people
working for US forces
The man displayed his Interior Ministry identification card and said he was a liaison officer with US forces. Behind the men was the black banner of al-Qaida in Iraq.
Ulaiywi said he provided the US military with the names "of officers of the former Iraqi army, who are Sunnis, and their addresses".
At the end of the video, Ulaiywi was blindfolded and appeared to be shot once in the head.
The authenticity of the video could not be verified.
An interim Interior Ministry official, who spoke on condition of anonymity, said Ulaiywi worked as a liaison officer between the interior and oil ministries and was kidnapped more than a month ago.