Health chiefs said about 263 women and children were among the dead as terrible images of blood-soaked corpses emerged. The death toll is expected to rise.
The strikes across Gaza left at least 600 people injured but Israel warned it was resuming attacks until Hamas releases all hostages.
It was described as the deadliest day in the region since the start of the war on October 7, 2023, and brings the death toll since then to 61,700.
Israeli PM Benjamin Netanyahu came under criticism from all sides over the attack, which came just over a fortnight after the ceasefire ended.
His own people, including former hostages, warned the new violence could lead to the death of the 59 remaining detainees in Gaza. But defence minister Israel Katz said: "The gates of hell will open in Gaza if hostages aren't released."
And Netanyahu warned: "This is just the beginning. Israel will, from now on, act against Hamas with increasing military strength."
Scenes of carnage emerged in the southern city of Khan Younis, where one local told how devastated relatives were turning up at hospitals holding their children’s remains. Abu Rizq said: "If you're in a Gaza hospital, you will see blood everywhere."
image [https://cdn.magzter.com/1583517504/1742347059/articles/oURMxrMNBDFYIc4be6sys/1478188281.jpg]
A strike on one Rafah home killed 17 members of one family, including at least 12 women and children, according to Gaza’s European Hospital.
Momen Qoreigeh, who lost 26 members of his family in the attacks, said: "Strikes were like in the early days of the war.
"There was a huge attack on our residential block.
"The attack killed so many people from my family, some of them we still haven't recovered from under the rubble."
Israel has already cut off all aid for Gaza. Save the Children regional director Ahmad Alhendawl said: "Children and families have barely caught their breath and are being plunged back into a horrifically familiar world of harm they can't escape. This new slaughter was on starved, besieged, defenceless families.
"This cannot be what world powers allow children to return to."
Since the ceasefire began on January 19, displaced Gazans have been returning to their homes, often finding rubble in their place.
UN humanitarian coordinator for Palestine Muhannad Hadi bra...