Some in the crowd stomped and spit on the bodies. A sixth corpse was tied to a motorcycle and dragged through the streets as people screamed, "Spy! Spy!"
The Hamas military wing, Izzedine al-Qassam, claimed responsibility in a large handwritten note attached to a nearby utility pole. Hamas said the six were killed because they gave Israel information about fighters and rocket launching sites. Hamas did not provide any proof of the alleged collaboration.
The killing came on the seventh day of an Israeli military offensive that has killed more than 120 Palestinians, both militants and civilians. Israel has launched hundreds of airstrikes, targeting rocket-launching sites, weapons caches and homes of Hamas activists, as Palestinians fired hundreds of rockets at Israel.
In selecting its targets for airstrikes, Israel relies on unmanned spy planes, or drones, but also on a network of Palestinian collaborators who feed information to their handlers from Israel's domestic Shin Bet security service.
Israeli defense officials say Palestinian informers have been recruited ever since it captured the West Bank, Gaza Strip and east Jerusalem in the 1967 Mideast War. Some are recruited with promises of work permits or money, while others are blackmailed into collaborating.
There is broad consensus among Palestinians that informers for Israel deserve harsh punishment, and it is rare to hear someone speak out against killings of alleged collaborators. Such public killings have been carried out in the West Bank and Gaza since the first intifada -- or uprising -- against Israeli occupation in the late 1980s.
Human Rights Watch and other international rights groups have condemned such extrajudicial killings, as has the Palestinian Center for Human Rights, a Gaza City-based group. Human Rights Watch says Hamas has also tortured suspected collaborators.
Tuesday's killings took place in Gaza City's Sheik Radwan neighborhood.
Witnesses said a van stopped at the intersection, where four masked men pushed the six accused informers out of the vehicle. Salim Mahmoud, 18, said the gunmen ordered the six to lie face down in the street and then shot them dead.
Another witness, 13-year-old Mokhmen al-Gazhali, said the informers were killed one by one, as he mimicked the sound of gunfire.
They said only a few people were in the street at first -- most Gazans have been staying indoors because of the Israeli airstrikes -- but the crowd quickly grew after the killings. Eventually several hundred men pushed and shoved to get a close look at the bodies, lying in a jumble on the ground. One man spit at the corpses, another kicked the head of one of the dead men.
"They should have been killed in a more brutal fashion so others don't even think about working with the occupation (Israel)," said one of the bystanders, 24-year-old Ashraf Maher.
One body was then tied by a cable to the back of a motorcycle and dragged through the streets. A number of gunmen on motorcycles rode along as the body was pulled past a house of mourning for victims of an Israeli airstrike.
In Israel's last major Gaza offensive four years ago, 17 suspected collaborators who fled after their prisons were hit in airstrikes were later shot dead in extra-judicial killings.
During the current offensive, Tuesday's killings brought to eight the number of suspected informers being shot dead in public. On Friday, the body of one alleged informer was found in a garbage bin, and another was shot dead in the street. Hamas claimed responsibility for both killings.
Since seizing Gaza in 2007, Hamas has executed four informers by firing squad, and about a dozen more are on death row in Gaza.
During Israel's direct occupation of the West Bank and Gaza, some informers openly cooperated with Israeli forces. For example, one informer in the West Bank town of Jericho displayed a photograph of Israel's army chief at the time on the wall of his office, in a defiant display of his allegiance.
After Israel pulled back troops from parts of the West Bank, he and others were given refuge in Israel. Other informers were evacuated from Gaza after Israel withdrew in 2005, but Israel is believed to have maintained a network there.
Human rights groups have alleged, for example, that Gaza medical patients seeking treatment in Israel are sometimes approached by the Shin Bet at the crossing into Israel.
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