At least 195 Palestinians, including women and children, have been killed in an Israeli aerial bombardment on Hamas security installations.
Israel launched air attacks across the besieged Gaza Strip on Saturday, threatening that further operations would be carried out.
Emergency services said that at least 300 people had been wounded.
Witnesses reported heavy damage as at least 30 missiles were fired.
Ehud Barak, the Israeli defence minister, said that the operation would not be short.
"The operation will go on and be intensified as long as necessary," he said on Saturday.
An Israeli military spokesman added that any "Hamas target is a target".
As dusk fell, Israel continued to bomb the strip, firing on a metal foundry in the south.Read More
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.
Hamas bases destroyed
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.
At least a dozen bodies of men in black uniforms were photographed at the Hamas police headquarters in Gaza City.Read More