Iraqi officials say seven people have been killed in attacks in the capital, Baghdad, and in the western city of Fallujah, including a mosque preacher who was killed outside a sweets shop.
A police officer said assailants killed on Sunday the preacher, Sheikh Talib Zuwayid of the Al-Baraa mosque in Fallujah, along with his nephew and another man.
A health official confirmed the deaths and said another man was injured.
Residents said Zuwayid was one of the organisers of weekly Sunni Muslim demonstrations against the Shia Muslim-led government of Nouri al-Maliki that have been taking place for the past three months.
The police officer, however, said it was unclear if the killings were related to the protests.
Immediately after he was killed, residents hung up posters announcing the man's death on Fallujah's main road.
The officials spoke on condition of anonymity because they were not authorised to release information.
In a separate attack, a Shia Muslim mosque came under attack in southern Iraq, when a bomb exploded during a service, wounding six people. The bomb exploded in the usually stable oil hub of Basra.
No one claimed responsibility for the attack, but Sunni Muslim fighters have stepped up efforts to undermine
the Maliki government.
In Baghdad, two separate explosions killed four people and wounded 14 more on Sunday.
Two police officers said a bomb attached to a policeman's car exploded early in the morning in the city's northern Kazimiyah neighbourhood, killing an officer and a bystander and wounding five others.
In another incident, two other police officers said a suicide bomber drove an explosives-laden car into an army checkpoint in Baghdad's western suburb of Abu Ghraib, killing two soldiers and wounding nine people, of which four were civilians.
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Two health officials confirmed the casualities.
All officials spoke anonymously, as they were not authorised to release information.
Police and military personnel are favourite targets of fighters seeking to undermine the Iraqi government's efforts to maintain security.
In Sunday's other violence, five shootings and bombings in Kirkuk and Mosul, both northern cities, killed one person and wounded six others.
All of the injured were policemen or soldiers.
The attacks come in the run-up to provincial elections scheduled for April 20, due to be held in 12 of Iraq's 18 provinces, the country's first polls since a parliamentary vote in March 2010.
But questions have been raised over the credibility of the polls as they have been postponed in two provinces hit by months of protests, and 11 candidates have been killed, according to an AFP news agency tally.
Although markedly lower than its peak in 2006 and 2007, levels of violence remain high in Iraq - at least 267 people have been killed in attacks this month, the highest figure since August 2012.