Hamas gunmen killed 18 Palestinians accused of spying for Israel, some of whom were lined up along the wall of a mosque after Friday prayers and shot in the head.
The Hamas-linked Al Majd website said the killings, which are the largest number of suspected informers ever killed by the militant group in a single day, were "choking the necks of the collaborators." It warned that suspects would not be dealt with through the justice system but "in the field".
The Palestinian Centre for Human Rights said two of those killed Friday were women and called for an immediate halt to what it said were "extra-judicial executions."
Crowds gather to watch the killing of seven Palestinians
Those women were two of 11 alleged informants shot dead at the Gaza City police headquarters. Seven more, their faces covered with bags, were killed by masked gunmen after midday prayers in front of worshipers at the al-Omari mosque. Each had a piece of paper pinned to his forehead, with his initial and the crime he was accused of committing.
The condemned men "had sold their souls to the enemy for a cheap price" one of the gunmen said, according to a source who spoke to the Associated Press.
The crackdown is said to have been sparked by the killing of three senior military leaders of Hamas in Israeli airstrikes, men Israel says are responsible for building its tunnel network to smuggle weapons. Earlier in the week, another strike killed the wife and two children of Mohammed Deif, the shadowy leader of the Hamas military wing. Deif's fate remains unclear.
Hamas said it would not release the names of those killed because it wanted to protect the reputation of their families.
In 2012, the killing of alleged "spies" by Hamas came to prominence when a mob set upon the bloodied corpses of six executed Palestinians. The bodies were stamped on and spat at, with one corpse tied to a motorcycle and dragged through the streets.
Masked gunmen forced the victims against the wall of a mosque
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 been carried out in the West Bank and Gaza since the first uprising against Israeli occupation in the late 1980s.
During Operation Cast Lead in 2008-9, 17 suspected collaborators were shot dead by Hamas.
Meanwhile, Israel-Gaza fighting continued for a third day since the collapse of Egyptian-led cease-fire talks earlier this week. By early afternoon, Gaza militants had fired at least 56 rockets and mortar shells at Israel, while Israel carried out at least 28 airstrikes in Gaza, the military said.
One of the strikes hit a livestock farm where two workers were killed and three people were wounded, said Gaza health official Ashraf al-Kidra. The Israeli military said its strikes targeted concealed rocket launchers and weapons sites.
In Israel, one civilian was moderately wounded by a rocket in the southern city of Beersheba and another was lightly hurt by a rocket that landed in the border town of Sderot.
Since Israel-Hamas fighting erupted on July 8, at least 2,091 Palestinians have been killed in the coastal territory, according to Gaza health official Ashraf al-Kidra.
Bags were put over the heads of the seven, killed in front of Friday worshippers
Nearly a quarter of the dead - 469 - are children, according to a UNICEF field officer in Gaza, Pernilla Ironside. Of the more than 10,500 Palestinians wounded, nearly one-third are children, according to UNICEF figures, while some 100,000 Gazans have been left homeless. On the Israeli side, 67 people have been killed in the past six weeks, including 64 soldiers, two civilians and a Thai worker.
The renewed fighting dashed hopes for a lasting truce. Earlier this week, Hamas rejected an Egyptian truce proposal under which Israel would gradually ease its blockade of Gaza, without giving specific commitments. Hamas demands a lifting of the border closure imposed by Israel and Egypt after the militant group's takeover of the coastal strip in 2007.
A quick resumption of indirect talks between Israel and Hamas in Cairo also seems unlikely, particularly after the killing of the three Hamas commanders. Senior Hamas political leader Ismail Haniyeh said late Thursday that his group would not budge from its demands.
"We will not accept anything less than an end to the aggression and an end to the blockade," Haniyeh said in a statement posted online by Hamas-run news service Al Rai. "Anyone involved in cease-fire efforts must understand that our people will not accept anything less than this."