In one incident, masked gunmen lined up seven men, their heads covered by bags, along a wall outside a Gaza City mosque and shot them to death in front of hundreds of people, witnesses said. A note pinned on the wall said they had leaked information about the location of tunnels, homes of fighters and rockets that were later struck by Israel.
In Israel, a 4-year-old boy was killed when a mortar shell hit two cars in the parking lot of Nahal Oz, a small farming community near Gaza. Five Israelis were hurt, one seriously, in several rocket strikes, the military said. One rocket damaged a synagogue.
The child's death was bound to raise pressure on Prime Minister Benjamin Netanyahu from an increasingly impatient public to put an end to rocket and mortar fire from Gaza - something Israel's military has been unable to do after 46 days of fighting with Hamas.
Netanyahu's office said he expressed his condolences and vowed that Hamas would pay a "heavy price."
The Israeli military said in a statement the deadly mortar shell had been fired from next to a U.N. school currently serving as a shelter for displaced Gazans, but then retracted its claim and said the facility in question was run by Hamas. Israel has repeatedly said Hamas uses schools, mosques and residential areas as cover from which to stage attacks, putting civilians at risk by drawing Israeli retaliation.
Chris Gunness, spokesman for the U.N. refugee agency UNRWA, said the agency strongly denied any claim that one of its facilities had been used in the deadly attack.
By early evening, Gaza militants had fired at least 117 rockets and mortar shells at Israel, while Israel carried out at least 35 airstrikes in Gaza, the military said. Palestinian officials in Gaza reported heavy Israeli activity overnight, with an additional 25 airstrikes by 1 a.m.
In a new warning from Israel's military, automated phone messages told Gaza residents that "Hamas has decided to go to war again" and that people must "get away immediately from areas where Hamas conducts terror activities."
At about the same time, an airstrike hit a house in Gaza City, and a huge orange ball of fire from the explosion rose into Gaza's night sky.
At least 45 people were wounded, health officials said. Ayman Sahabani, head of the emergency department at the city's Shifa Hospital, said only a minute passed before the firing of a warning missile and the bombing.
Early Saturday, an Israeli airstrike hit a house in central Gaza, killing a 47-year-old woman and wounding at least seven other people, said Gaza health official Ashraf al-Kidra. Rescuers were searching the rubble for more survivors.
Since Israel-Hamas fighting began July 8, at least 2,092 Palestinians have been killed, al-Kidra said. According to U.N. figures, at least 478 Palestinian children and minors were among the dead, including 320 who were 12 or younger.
On the Israeli side, the boy's killing Friday raised the death toll to 68, including 64 soldiers, three civilians and a Thai worker.
Friday's escalation came three days after Israel-Hamas truce talks collapsed in Cairo.
At the talks, Hamas had rejected an Egyptian proposal under which Israel would gradually ease its border blockade of Gaza in exchange for a period of extended quiet. Hamas said Israel offered nothing specific, and says it will only halt fire if Israel and Egypt agree to open Gaza's borders to trade and travel.
The border blockade was imposed in 2007, after the violent Hamas takeover of Gaza. Israel has said it cannot lift the closure unless Hamas stops trying to smuggle or manufacture weapons and agrees to disarm, a demand the Islamic militants have rejected.
Mahmoud Zahar, a Hamas leader in Gaza, signaled Friday that Hamas would also reject any U.N. Security Council resolution that seeks to disarm the group. Britain, Germany and France are working on a cease-fire resolution that calls for opening Gaza's borders, in exchange for restoring the rule of Western-backed President Mahmoud Abbas' Palestinian Authority in Gaza. Abbas lost control of Gaza in the Hamas takeover.
Zahar wrote in a text message that "Hamas will not accept any international resolutions that try to touch the weapons of the resistance and don't fully lift the blockade."
Abbas, meanwhile, met with the top Hamas leader in exile, Khaled Mashaal, to push for Hamas to accept the Egyptian cease-fire proposal. The talks took place in Qatar, a regional backer of Hamas, but it was not clear if Abbas made headway. He left Friday for Egypt, ahead of talks with the Egyptian president Saturday.
In Gaza, the killing of the 18 alleged spies for Israel followed the deaths of three top Hamas military commanders in an Israeli airstrike, reportedly with the help of local informants.
Even after Israel's withdrawal from Gaza in 2005, Israel's Shin Bet security service continued to run a network of informers, recruiting them through blackmail, the lure of exit permits or money.
Hamas media said the shootings signaled the start of a new crackdown, under the rallying cry of "choking the necks of the collaborators." It was the largest number of suspected informers killed in a single day under Hamas rule.
The Al Majd website, which is close to the Hamas security services, said suspects would now be dealt with "in the field" rather than in the courts in order to create deterrence. Hamas said it would not release the names of those killed because it wanted to protect the reputation of their families.
In the morning, 11 alleged informants were shot by a firing squad at Gaza City police headquarters. Two of the 11 were women, according to the Palestinian Center for Human Rights. The group urged an immediate halt to what it called "extrajudicial executions."
Later in the day, seven people were killed outside the downtown al-Omari mosque as worshippers wrapped up noon prayers. Photos from the incident posted on several Palestinian websites showed several hundred people gathered near the scene.
The photos showed masked, black-clad gunmen leading several men with bags over their heads to a wall.
Witness Ayman Sharif, 42, quoted one of the gunmen as saying the seven "had sold their souls to the enemy for a cheap price" and had caused death and destruction. The commander of the group then gave the order to the others to open fire with automatic rifles. He said the bodies were collected by an ambulance and the gunmen left.
The Hamas-run Al Rai news website said that after the public killings, several suspected collaborators "surrendered to the resistance and declared that they repented." It did not provide details.
Friday's killing marked the third time since the outbreak of the Israel-Hamas war six weeks ago that Hamas announced the killing of alleged collaborators. On Thursday, Al Majd said seven people were arrested on suspicion of working with Israel and that three of them were killed.
Alhlou reported from Jerusalem. Associated Press writers Ibrahim Barzak in Gaza City and Josef Federman in Jerusalem contributed to this report.