Seven of those killed were found decapitated and dumped north of Baghdad in what appeared to be a sectarian revenge killing, police said on Saturday.
The decapitated bodies were found in an orchard in the city of Duluiyah, 75km north of Baghdad.
Three of the corpses had been among a group of 17 construction workers kidnapped on Thursday while travelling home to the predominantly Shia town of Balad, police said.
The corpses of the other 14 workers were found earlier on Friday, also all beheaded.
The killings were apparently in retaliation for Wednesday's kidnapping and killing of three Sunni Arabs in Duluiyah district by a Shia militia based in Balad, police said.
In central Baghdad, two bombs exploded in a car park in the Rasheed district shortly after mid-day, killing one person and injuring four others, Lieutenant Bilal Ali said.
In Diwaniyah, 130km south of Baghdad, Mohammed Muhsin al-Marmadhi, a teacher, was killed in a drive-by shooting as he was leaving his home, police said.
A family of 10, including five women and three children, in Mahmoudiya, perished at the hands of unidentified assailants dressed in military-style uniforms who stormed into their house around dawn.
Seven other people were killed in a mortar attack near Baquba, police said.
In Baghdad, a journalist working for government-run TV was killed in a similar drive-by shooting in southern Baghdad on Friday night, police said.
Raed Qais al-Shammari, a technician with the al-Iraqiya station, had been standing near his home talking with a friend when he was shot by an unidentified assailants from a car in the violence-wracked Dora neighbourhood.
The bodies of two unidentified men were also fished out from the Tigris river in Suwayrah, 40km south of Baghdad, Mohammed al-Shammari, police lieutenant said.
The men had been blind folded with their hands and legs bound and were shot in the heads, al-Shammari said.