You are not permitted to download, save or email this image. Visit image gallery to purchase the image.
Militants wearing masks and dressed in black gunned down seven of those condemned, whose faces were covered and hands bound, in front of worshippers emerging from the Omari mosque on Palestine Square, in the first public executions in the enclave since the 1990s.
A further 11 were killed at an abandoned police station near Gaza City, Hamas security officials said.
Two bodies were seen being loaded onto an ambulance before Reuters journalists were told to leave the area.
"The resistance has begun an operation called 'strangling the necks', targeting collaborators who aid the (Israeli) occupation, kill our people and destroy houses," said Al-Majd, a website run by Hamas's internal security service.
A so-called conviction letter signed by the "Palestinian Resistance" was posted on a wall near where the bodies of the alleged collaborators lay. The notice read:
"They provided the enemy with information about the whereabouts of fighters, tunnels of resistance, bombs, houses of fighters and places of rockets, and the occupation bombarded these areas, killing a number of fighters ... Therefore, the ruling of revolutionary justice was handed upon him."
The crackdown on suspected collaborators follows the killing of three of Hamas's most senior military commanders in an Israeli air strike on Thursday. That attack depended on precise intelligence on their whereabouts.
The Palestinian Center for Human Rights denounced the killings of Gazans by militants.
"We demand the Palestinian National Authority and the resistance (Palestinian armed factions) to intervene to stop these extra-judicial executions, no matter what the reasons and motives are," Raji al-Surani, the chairman of the organisation, said in a statement.
Fighting between Israel and Hamas raged on. A mortar bomb fired from Gaza killed a four-year-old Israeli boy, the first Israeli child killed in the conflict.
More than 80 rockets were fired from Gaza into Israel, wounding three people at a synagogue in Ashdod and two others in Beersheba and Sderot, Israeli police said.
Israeli forces carried out more than 25 air strikes in Gaza on Friday, killing four people, Palestinian health officials said.
Israel launched its offensive on Gaza on July 8 with the stated aim of putting an end to cross-border rocket fire.
More than 2070 Palestinians have since been killed in the conflict, most of them civilians, while large areas of Gaza have been destroyed, leaving around 400,000 of the enclave's 1.8 million people displaced. Sixty-four Israeli soldiers and four civilians in Israel have been killed.
HAMAS LEADERS ARE TARGETS
Prime Minister Benjamin Netanyahu has praised cooperation between Israel's military and its internal security service, Shin Bet, for the air strike that killed the three Hamas commanders, saying it demonstrated a high level of knowledge about the whereabouts of Hamas's leaders.
Hamas identified the three as Mohammed Abu Shammala, Raed al-Attar and Mohammed Barhoum, saying they had been at the forefront of the fight against Israel for two decades.
Israel said two of them had been instrumental in the 2006 kidnapping of an Israeli soldier who was held in Gaza for five years before being freed in a prisoner exchange, as well as other deadly attacks.
Since a 10-day ceasefire collapsed on Tuesday, Israel has focused its attacks on the military leadership of Hamas.
On Tuesday, as the Egyptian-brokered ceasefire fell apart, an Israeli air strike hit a building in northern Gaza, killing the wife and two children of Mohammed Deif, Hamas's top military commander.
Deif, mastermind of a tunnel network under Gaza that has been used to attack Israel and long a target, appears to have survived, although his whereabouts are unknown.
On Thursday, Netanyahu granted preliminary approval for the call-up of 10,000 army reservists, signalling the possibility of heightened military action in Gaza.