Sheikh Khaled al-Jumaili, widely seen as the protest leader in an ongoing sit-in in Fallujah, around 60 kilometers northwest of the Iraqi capital Baghdad, was gunned down by armed men driving a private car as he was driving on the way to the sit-in camp, the source added.
Al-Jumaili's son, who was beside him in the car, was also seriously wounded in the attack.
The security source added that policemen had headed to the site of the attack, took al-Jumaili's body to the forensic agency and started an investigation into the attack. He said another police force started searching for the perpetrators.
Nobody claimed responsibility for the assassination.
Sunni Iraqis have been staging sit-ins in several provinces since last December to denounce what they claim as discrimination of the government of Prime Minister Nouri al-Maliki against Sunnis.