The civil defence agency in Gaza said three Israeli air strikes killed more than 40 people within an hour across the Palestinian territory.
Civil defence spokesman Mahmud Bassal said at least 44 people were killed and dozens more wounded in the three strikes, described as "massacres".
The Gaza health ministry said at least 17 people died and 26 were wounded close to a petrol station near the southern city of Khan Younis.
The Palestinian Red Crescent said five died at the UN-run Al-Razi School in the Nuseirat refugee camp in central Gaza.
The third strike was on a gathering of people near a roundabout in northern Gaza, according to the civil defence agency, which did not give a precise toll breakdown.