The man, Ali Haddad, was shot to death shortly before a meeting between Palestinian President Mahmoud Abbas and Palestinian Prime Minister Ismail Haniyeh was scheduled to begin.
According to eyewitnesses, gunmen stopped the car in which Haddad was traveling, opened fire and killed him. Nine people were injured in the shooting, one of them sustaining critical wounds.
Hamas' military wing accused Fatah members serving in the Preventive Security Service of being responsible for the murder.
Abu Uabida, a spokesman for the Izz al-Din al-Qassam Brigades, said that the killing would not be ignored.
"We will obtain revenge and will not allow these murderers to enjoy immunity from their party or family," he warned, adding that all those who took part in the shooting would be sorry for their involvement.
Another member of Hamas' military wing was killed Sunday. The two incidents are expected to heighten tensions between the two factions.
Palestinian sources told Ynet that Haddad was accused several months ago of being responsible for the killing of an officer in the Preventive Security Service in response to the killing of a senior Hamas member.