Palestinian President Mahmoud Abbas has appealed to all factions to stop the "madness" of the fighting in Gaza.
The clashes between Hamas and Fatah militants have left dozens of people dead.
Mr Abbas, who leads Fatah, warned the violence could bring Gaza to the point of collapse.
Hamas men appeared to be systematically taking over Fatah-controlled buildings throughout Gaza.
An international human rights group accused both factions of committing serious violations of international humanitarian law.
Palestinians in the West Bank also appealed to Mr Abbas to stop the fighting.
The fighting then spilled over into the West Bank, with a gun battle breaking out in the northern town of Nablus.