At least 38 Palestinians were killed in a series of Israeli air strikes across the Gaza Strip during a half-hour period overnight, according to Palestinian medics.
Israeli warplanes struck several locations, including a school in eastern Gaza City, and an abandoned fuel station sheltering displaced families in central Gaza's Nuseirat refugee camp, according to the Hamas-run civil defence agency.
It added that rescue teams were facing major difficulties reaching the wounded due to a lack of equipment and the intensity of the bombardment.
The attack comes after Israel issued a sweeping evacuation order for civilians in Gaza in preparation for what it said was an "unprecedented attack".
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Ten people were killed when an Israeli air strike hit classrooms at the Musa bin Nusayr School, where hundreds of displaced people were sheltering, according to local activists in Gaza City.
In central Gaza, 15 people were reported killed in a strike targeting tents at an abandoned fuel station in Nuseirat.
Meanwhile, local journalists said 13 Palestinians were killed in an air strike on the Abu Samra family's three-story home in central Deir al-Balah.
Separately, Israeli jets launched 10 air strikes on the Shujaiya neighbourhood in eastern Gaza City.
Local reports described the blasts as causing tremors similar to an earthquake. The strikes are believed to have targeted Hamas's network of underground tunnels.
The attack comes as the leaders of the UK, France and Canada issued a joint statement threatening action if Israel Prime Minister Benjamin Netanyahu persisted with his goal to "take control" of all of Gaza.