A doctor at the Shifa hospital in Gaza City described a desperate scene, with essential medical supplies, fuel and water all running out.
Dr Khamis El Essi told the BBC that many people had been admitted with multiple injuries, such as head and back wounds. Some patients had had limbs amputated.
He said some of the injured were carers and relatives who had been sheltering in a nearby mosque when it was hit in an Israeli raid.
The air strikes were launched on Saturday against Hamas targets in the densely-populated coastal territory, less than a week after the expiry of a six-month-long ceasefire deal with the militant group.
Israel hit targets in all Gaza's main towns, including Gaza City in the north and Khan Younis and Rafah in the south.
More than 210 targets were hit in the first 24 hours of what Israel says could be a lengthy military operation.
Israeli Foreign Minister Tzipi Livni on the Gaza assault
"We took Hamas by surprise, we targeted Hamas headquarters, so this is the beginning of a successful operation, I hope, but the idea is to change realities on the ground," Foreign Minister Livni told the BBC on Sunday.
"We are trying to give an answer to our own citizens who want to live in peace."
The high numbers of casualties made Saturday the single deadliest day in the Gaza Strip since Israel's occupation of the territory in 1967, analysts said, although no independent confirmation is available of the numbers killed.
Most of those killed were policemen in the Hamas militant movement, which controls Gaza, but officials said women and children also died.
The head of Gaza's police was among those killed.
Up to 700 others were wounded as missiles struck security compounds and militant bases, the officials added.
RUN-UP TO GAZA RAIDS
Shaky Egyptian-brokered six-month truce between Hamas and Israel agreed on 19 June
Palestinian rocket fire and Israeli operations in Gaza reduced
Violence resumes on 4 Nov as Israel launches incursion which it says targets Hamas tunnel; Palestinians respond with rocket fire
Hostilities increase; Israel tightens blockade on Gaza
Truce expires on 19 Dec; both sides blame each other for its breakdown; rocket fire increases
The Egyptian foreign minister has accused Hamas of not allowing injured Palestinians to leave Gaza to seek treatment, even though much-needed medical supplies are waiting at the nearby El-Arish airport.
In Israel, one person was killed on Saturday in the town of Netivot, some 20km (12 miles) east of Gaza, while there were reports of several Qassam rocket strikes early on Sunday.
Rockets landed in Ashdod, Israel's largest southern city - some 38km (23 miles) from Gaza - the deepest they have ever struck inside Israel, Israeli media said. No injuries were reported.
A Palestinian youth was killed by Israeli fire in the north of the West Bank during protests against the raids, medics said.
In Gaza, Palestinian officials said two people died when a mosque was hit on Saturday night.
A BBC journalist in Gaza City said a Hamas-run security and prison compound was hit by at least three missiles on Sunday morning. Hamas said all of its security compounds in the strip were destroyed on Saturday.
'Time for fighting'
At the UN, the Security Council ended emergency talks with a call for an end to hostilities, speaking of "serious concern" at the escalation of the situation in Gaza.
Israel has positioned tanks just outside the Gaza Strip
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