At least eight people have died in factional fighting in the Gaza Strip in the past 24 hours, reports say.
Fatah is marking the anniversary of its foundation in 1965
Most of those killed are believed to be supporters of rival Palestinian groups Fatah and Hamas, but one elderly man died after being caught in crossfire.
The clashes are also reported to have involved members of other groups, including the powerful Doghmush clan.
On Monday, President Mahmoud Abbas urged Hamas to give up its control of Gaza, and resume dialogue with Fatah.
The official Palestinian news agency says Hamas men have detonated explosives inside a six-storey building in Gaza City owned by members of the Doghmush family - an influential Gaza clan said to have links to armed groups.
The agency says the building is now close to collapse.
Monday's fighting erupted when Fatah tried to hold a rally to mark the anniversary of its foundation in 1965 on 1 January.
In another development in the Gaza Strip, one Hamas member was killed and five others were injured by Israeli tank fire.
The Israeli army said soldiers had opened fire after seeing gunmen approach the border fence with Israel.