Gunmen have opened fire at a convoy carrying the acting Palestinian intelligence chief, hitting him the chest and causing his car to overturn.
It appeared to have been a well-planned attack
Brig-Gen Tariq Abu Rajab was brought to Shifa hospital in Gaza, before being transferred to an Israeli hospital.
Two bodyguards were killed and two other people were wounded.
Gaza has seen a wave of inter-Palestinian violence in recent months, as various factions vie for power ahead of Israel's planned withdrawal.
Mr Abu Rajab's convoy was attacked near the Shati refugee camp in the west of Gaza city, as it headed towards his local headquarters.
Palestinian hospital officials initially said Mr Abu Rajab was in stable condition and his injuries were not life-threatening.
However, he was then transferred to a hospital in the southern Israeli city of Ashkelon, where he was immediately taken to surgery.
One of the other injured was said to be in a serious condition. The dead bodyguards were named as Khamis Abdul Jawad and Samir Hijjo.
The 58-year-old has been acting as head of the General Intelligence Service since the top commander Amin Hindi resigned in July after a string of high-profile kidnappings.