* At least 18 suspected collaborators killed in Gaza
* Seven of those killed publicly executed near main mosque
* Israel kills three Palestinians in air strikes on Gaza
* Gaza militants fire more than two dozen rockets at Israel
By Nidal al-Mughrabi and Allyn Fisher-Ilan
GAZA/JERUSALEM, Aug 22 (Reuters) - Hamas-led gunmen in Gaza executed 18 Palestinians accused of collaborating with Israel on Friday, a day after Israel tracked down and killed three top Hamas commanders, the highest-ranking militants to be killed in the six-week war.
Seven people were shot dead in front of worshippers outside a mosque in one of Gaza's main squares, witnesses said, the first public executions in the Palestinian enclave since the 1990s. A further 11 were killed at an abandoned police station near Gaza City, Hamas security officials said.
In the public execution, militants wearing masks and dressed in black gunned down the suspects, whose faces were covered and hands bound, as worshippers emerged from the Omari mosque on Palestine Square, one of Gaza's busiest districts.
"The resistance has begun an operation called 'strangling the necks', targeting collaborators who aid the (Israeli) occupation, kill our people and destroy houses," a pro-Hamas website said.
A so-called conviction letter signed by the "Palestinian Resistance" was posted on a wall near to where the bodies of 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,"
Earlier on Friday, 11 suspected collaborators were shot dead at an abandoned police station, a Hamas security official said. At the site, Reuters saw two bodies being loaded onto an ambulance before being told to leave the area.
The Palestinian Center for Human Rights in Gaza denounced the killings.
"We demand the Palestinian National Authority and the resistance (Palestinian armed factions) to intervene to stop these extra-judicial executions, no matter what reasons and the motives are," Raji al-Surani, the chairman of the organisation, said in a statement.
Israel launched an offensive on July 8 with the stated aim of putting an end to cross-border rocket fire from Gaza.
Health officials in the small, densely populated enclave said the Palestinian death toll rose to 2,070 on Friday, mostly civilians, after a father and his son were killed in an Israeli air strike near Khan Younis in southern Gaza.
Sixty-four Israeli soldiers and three civilians in Israel have also been killed in the conflict.
The executions followed the killing by Israel of three of Hamas's most senior military commanders, who were hit in an air strike on a house in the southern city of Rafah on Thursday.
Israeli Prime Minister Benjamin Netanyahu has praised cooperation between Israel's military and its internal security service, Shin Bet, for the attack, which demonstrated a high level of knowledge about the whereabouts of Hamas's leaders.
Hamas, which has dominated Gaza since seizing power there in 2007, fired more than 25 rockets into Israel on Friday, injuring two people, Israel's emergency services said. The Israel air force carried out more than 25 air strikes across the Gaza Strip, killing three men, Gaza medics said.
Since a 10-day ceasefire collapsed on Tuesday, Israel has focused its attacks on the military leadership of Hamas.
Hamas identified those killed on Thursday 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.
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.
Attacks by Israeli artillery and air strikes have devastated many areas of Gaza, a densely populated enclave that is home to 1.8 million people. The United Nations says at least 425,000 people have been displaced by the fighting.
While the intensity of rocket fire has diminished somewhat this week, Netanyahu remains under heavy pressure domestically to go further to combat the threat. As well as areas close to Gaza, the rockets have targeted Israel's business centre Tel Aviv and near Ben Gurion airport, its international hub.
On Thursday, Netanyahu granted preliminary approval for the call-up of 10,000 army reservists, signally the possibility of heightened military action in Gaza. But any such move would increase the risk to Israeli troops and civilians in Gaza. (Writing by Allyn Fisher-Ilan; Editing by Luke Baker and Mark Heinrich)