According to Palestinian sources in the Gaza Strip, most of the gunmen who have been fighting the IDF over the past few days are members of Hamas's armed wing, Izaddin Kassam.
"At least 2,000 Hamas gunmen have been deployed in the northern Gaza Strip to take part in the fighting," the sources told The Jerusalem Post.
The sources estimated that Izaddin Kassam has at least 15,000 members divided into four brigades in the Gaza Strip.
They added that the Hamas gunmen were using many American-made arms seized from the Fatah-controlled Palestinian Authority security forces in June.
Hamas says it captured thousands of M-16 and Kalashnikov rifles, and large supplies of ammunition during its weeklong conquest of Gaza. Hamas is also believed to have acquired weapons capable of penetrating armor and stockpiles of rocket-propelled grenades.
A senior Hamas official said Sunday that his movement had smuggled hundreds of rockets and mortars and tons of explosives into the Gaza Strip from Egypt in the past few months.
The Hamas-dominated police force in the Gaza Strip, which has around 15,000 members, has not been involved in the fighting, although the IDF has targeted some of its bases.
Hamas has been careful not to send the police force to the battlefield so as to avoid a total breakdown of its official institutions. The policemen are needed by Hamas to maintain law and order and to thwart any attempt by rival groups such as Fatah from taking advantage of the security deterioration to topple the Hamas government.
Other groups that are involved in the fighting include Islamic Jihad, the Popular Resistance Committees and some splinter factions belonging to Fatah's armed wing, the Aksa Martyrs Brigades.
These three groups, which according to Palestinian sources are operating in coordination with Hamas, have also been behind many of the rocket attacks on Israel in the past few days. Altogether, the three groups have fewer than 1,500 gunmen in Gaza.
But Hamas has sought to play down the role of the other armed groups in the fighting in the hope that it will score points on the "Palestinian street" as the major force that fought "courageously" against Israel. Hamas is hoping that once the fighting is over, it will be able to declare "victory, as Hizbullah did after the war in 2006."