Hamas gunmen executed 11 Palestinians suspected of collaborating with Israel at an abandoned police station in Gaza earlier in the day, Hamas security officials said.
Shortly afterward Hamas shot seven more Palestinians in front of hundreds of people in a central Gaza square, witnesses said.
Hamas warned the same punishment will be imposed soon on others.
The executions came a day after Israel killed three top Hamas commanders - Mohammed Abu Shamala, Mohammed Barhoum and Raed al-Attar who were key to Hamas operations including smuggling, tunnel construction and also played a role in capturing Israeli soldier Gilad Shalit in 2006.
Hours after the targeted strike on the Hamas commanders, Ismail Haniyeh, the former head of the Hamas regime in Gaza, said, "After a senior operative is killed, we immediately continue on our path without hesitating or stepping back".
Hamas, the militant group that rules Gaza, has warned that Israel will "pay the price" for killing the high-ranking leaders of its military wing, the Qassam Brigades.
Seven civilians were also killed in the bombing of a house in southern Gaza where the leaders were located.
The IDF said the military leaders were responsible for "major terror attacks against Israelis."
In Israel, the barrage of Palestinian rocket attacks claimed its fourth civilian victim of the entire war - a boy killed near a kindergarten in the southern community of Sdot Negev.
Following the death, Israeli Prime Minister Benjamin Netanyahu vowed harsh retribution against Hamas.
"Hamas will pay a heavy price for this attack," Netanyahu's spokesman Ofir Gendelman cited the premier as saying on his Twitter account.
Meanwhile, two Palestinians, aged 24 and 22, were killed in an Israeli airstrike on Nusseirat refugee camp early today, emergency services said.
A drone strike later targeted a 45-year-old man in Nusseirat who died of his injuries in hospital shortly afterwards.
Two more were killed in an air raid near neighbouring Deir al-Balah.
Ashraf el-Qedra, spokesman for the Ministry of Health in Gaza, said 72 Palestinians have been killed since the hostilities resumed on Tuesday after the collapse of ceasefire talks in Cairo aimed at finding a lasting end to the fighting.
Palestinian militants fired more rockets into Israel, and the Israeli Defence Forces kept up airstrikes on Gaza.
El-Qedra said 2,087 Palestinians have been killed since the conflict began in early July. The UN estimates that around 70 per cent of the dead were civilians. The fighting has killed 67 Israelis, almost all of them soldiers.
The truce fell apart after Israel reported that militants had started firing rockets again.
Since the fighting flared up again this week, 360 rockets have been fired from Gaza toward Israel, according to the Israel Defence Forces.
They have caused some injuries but no deaths. Israel's Iron Dome missile defence system has intercepted 56 of the rockets, the IDF said.
The Israeli military has attacked around 200 targets in Gaza during the same period, the IDF said.