The Hamas-run health ministry in Gaza says an Israeli raid on a refugee camp - which led to the rescue of four hostages - killed 274 people, including children and other civilians.
On Saturday Israel's forces, backed by air strikes, fought intense gun battles with Hamas in and around the Nuseirat refugee camp, freeing the captives.
Noa Argamani, 26, Almog Meir Jan, 22, Andrei Kozlov, 27, and Shlomi Ziv, 41, who were abducted from the Nova music festival on 7 October have been returned to Israel.
The Israeli military has estimated that fewer than 100 people died in the operation.
But the latest figures from the Hamas-run health ministry in Gaza would, if confirmed, make it one of the deadliest days of the conflict so far.
People living in the densely-populated area have described the terror of coming under intense bombardment and heavy gunfire.
One man, Abdel Salam Darwish, told the BBC he was in a market buying vegetables when he heard fighter jets from above and the sound of gunfire.
"Afterwards, people's bodies were in pieces, scattered in the streets, and blood stained the walls," he said.
The return of the hostages to their families has sparked celebration in Israel and world leaders, including US President Joe Biden, have welcomed the news of their release.
But there has been mounting criticism of the deadly cost of the operation inside Gaza, with European Union foreign affairs chief Josep Borrell saying he condemned it "in the strongest terms".
"Reports from Gaza of another massacre of civilians are appalling," he wrote on X.
An Israeli minister said that instead of condemning Hamas for hiding behind civilians, the EU had condemned Israel for saving its citizens.
Two hospitals in Gaza, al-Aqsa hospital and al-Awda hospital, said they had counted 70 bodies between them.
The Hamas-run health ministry released names of 86 people out of the 274 Palestinians it says were killed during the two-hour operation.
Previously, Israel's military spokesman Daniel Hagari estimated there were fewer than 100 casualties in what was a "high-risk, complex mission" based on "precise intelligence".
Defence Minister Yoav Gallant said special forces operated "under heavy fire" when rescuing the hostages. One special forces officer was wounded and later died in hospital.
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