A bomb was set off near an Iraqi military convoy in Mahmudiya, killing five people and wounding 15, hospital sources said.
A doctor added that four soldiers were among the dead and that three of the wounded were in critical condition.
The 8.30am (0530 GMT) attack was one of a series of bombings that focused on the northern city of Kirkuk.
Three polling stations and a US military convoy were also targeted.
According to the Iraqi police force's General Sarhad Qadir on Thursday, the first bomb exploded at 8.10am near the police academy as a US military convoy was passing.
The car bomb in Baquba killed
at least one person
He said there were no casualties or serious damage. A polling station was also bombed about 10km north of Kirkuk and two more in the east of the city were damaged.
The general said two civilians were wounded on Wednesday night by mortar shells that fell near a police station at Kaza, about 10km south of the city.
Farther south in Tikrit, a bomb exploded in front of a police car on Wednesday night, killing one police officer and wounding another.
In Samarra, several people were killed by a car bomb. An Iraqi police official said seven had died in the attack but a US military officer said the toll was three.
The bomber blew up the car as an army patrol passed near a polling station in the centre of Samarra, north of Baghdad, Lieutenant-Colonel Mahmud Muhammad of the Iraqi police said.
"Seven Iraqis, including four soldiers, were killed and five others - two soldiers and three civilians - were wounded," he said.
Bombers targeted an Iraqi army
patrol in Samarra
But a US officer said the car bomb killed an Iraqi soldier and two civilians.
Doctors at the local hospital said four Iraqi soldiers and two civilians were wounded in the blast.
The dead civilians were a man and his son who ran a fruit and vegetable stall. A large crater was blown in the road and fruit scattered around the blast site.
US, Australian toll
A US soldier from the 1st Infantry Division was shot dead early Thursday morning at a forward operating base near the town Muqdadiya in Diyala province. The military statement did not elaborate on the circumstances.
Elsewhere, eight Australian soldiers were wounded in a car bomb attack in Baghdad.
The barracks housing Australia's military contingent also came under sniper attack, defence department officials said on Thursday.
A school converted into a polling
station was destroyed
Defence Minister Robert Hill said a convoy of three armoured vehicles was transporting Australian officials along a Baghdad road known as Route Irish when it was attacked by what was believed to be a car bomb.
He said eight soldiers were hospitalised with wounds ranging from lacerations to broken bones. The two most seriously injured would be airlifted to Germany for further treatment.
Both the car bomb and sniper attacks occurred at about 10am Baghdad time on Wednesday.
Just hours later, a car bomb exploded next to the governor's offices in Baquba, killing one person.
According to photojournalist Nikola Solic, the explosion set several cars on fire and threw a body into the middle of the street.
Northeast of the capital, politically motivated violence has increased in recent weeks.
On Wednesday, the offices of three political parties were attacked and one police officer killed.