Witnesses who declined to be identified told Reuters at the scene the gunmen had eaten breakfast in a nearby restaurant while waiting for al-Farah to arrive.
The drive-by shooting came two days after the killing of three young children of a Fatah-allied Palestinian intelligence officer.
That incident prompted renewed clashes between the rival Hamas and Fatah factions and has raised the spectre of a civil war among Palestinians.
A senior official from Fatah has said the Hamas government bore responsibility for Monday's unprecedented attack on the children. Hamas has denied that it was involved.
Hamas blamed the Fatah for the latest killing.
"Lots of nations elect governments to clean up corruption, but people don't have to give up their policies to form a unity government"
Yun, Melbourne, Australia
Send us your views
"The seekers of the coup in Fatah bear the responsibility for all actions of chaos taking place in the Palestinian streets," Mushir al-Masri, a Hamas member of parliament, said.
Tawfiq Abu Khoussa, a Fatah spokesman, criticised Hamas for blaming the once-dominant faction.
"The brothers in Hamas must be accurate and not throw quick accusations before an investigation has yet to start," he said.
Ahmad al-Khalidi, the Hamas-appointed Palestinian interior minister, also denounced al-Farah's murder.
"Befeficiaries of such a state are the Israeli occupation and those who seek to topple this democratic Hamas government to show the governmet that cannot control the situation or achieve security and safety of the citizens," he told Al Jazeera.