Israeli F-16 bombers have pounded key targets across the Gaza Strip, killing more than 200 people, local medics say.
Most of those killed were policemen in the Hamas militant movement, which controls Gaza, but women and children also died, the Gaza officials said.
About 700 others were wounded, as missiles struck security compounds and militant bases, the officials said.
Israel said it was responding to an escalation in rocket attacks from Gaza and would bomb "as long as necessary".
They were the heaviest Israeli attacks on Gaza for decades. More air raids were launched as night fell.
The operation came days after a truce with Hamas expired.
Israeli Defence Minister Ehud Barak said "it won't be easy and it won't be short".
"There is a time for calm and a time for fighting, and now the time has come to fight," he said.
UN Secretary-General Ban Ki-moon called for an immediate halt to the violence, condemning what he called Israel's "excessive use of force leading to the killing and injuring of civilians" and "the ongoing rocket attacks by Palestinian militants".
Middle East envoy Tony Blair and the French EU presidency also urged an immediate ceasefire.
Palestinian militants frequently fire rockets against Israeli towns from inside the Gaza Strip; large numbers of rocket and mortar shells have been fired at Israel in recent days.
In a statement, Israel's military said it targeted "Hamas terror operatives" as well as training camps and weapons storage warehouses.
A Hamas police spokesman, Islam Shahwan, said one of the raids targeted a police compound in Gaza City where a graduation ceremony for new personnel was taking place.
"Hamas will continue the resistance until the last drop of blood"
Fawzi Barhoum
Hamas spokesman
At least a dozen bodies of men in black uniforms were photographed at the Hamas police headquarters in Gaza City.
Israel said operations "will continue, will be expanded, and will deepen if necessary".
It is the worst attack in Gaza since 1967 in terms of the number of Palestinian casualties, a senior analyst told the BBC in Jerusalem.
Israeli Foreign Minister Tzipi Livni defended the air raids, saying Israel had "no choice". "We're doing what we need to do to defend our citizens," she said in a television broadcast.
Israel hit targets across Gaza, striking in the territory's main population centres, including Gaza City in the north and the southern towns of Khan Younis and Rafah.
Hamas said all of its security compounds in Gaza were destroyed by the air strikes, which Israel said hit some 40 targets.
Mosques issued urgent appeals for people to donate blood and Hamas sources told the BBC's Rushdi Abou Alouf in Gaza that hospitals were soon full.
In the West Bank, Palestinian Authority President Mahmoud Abbas - whose Fatah faction was ousted from Gaza by Hamas in 2007 - condemned the attacks and called for restraint.
But Hamas quickly vowed to carry out revenge attacks on Israel in response to the air strikes, firing Qassam rockets into Israeli territory as an immediate reply.
One Israeli was killed by a rocket strike on the town of Netivot, 20 kilometres (12 miles) east of Gaza, doctors said.
"Hamas will continue the resistance until the last drop of blood," spokesman Fawzi Barhoum was reported as saying.
The air strikes come amid rumours that an Israeli ground operation is imminent.
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