Many of Israel's Western allies urged restraint on both sides, though the U.S. blamed Hamas for the fighting.
The offensive began eight days after a six-month truce between Israel and the militants expired. The Israeli army says Palestinian militants have fired more than 300 rockets and mortars at Israeli targets over the past week, and 10 times that number over the past year.
Streets were empty in Gaza City on Sunday as most residents stayed home, fearing more airstrikes. A few lined up to buy bread outside two bakeries. Schools were shut for a three-day mourning period the Gaza government declared Saturday for the campaign's dead.
Hamas police kept a low profile, wearing jackets over their dark blue uniforms and walking close to walls, hoping to evade the detection by Israeli pilots.
Aircraft struck one of Hamas' main security compounds in Gaza City _ a major symbol of the group's authority. Health officials said four people were killed and 25 wounded in the attack.
A column of black smoke towered from the building, and some inmates of the compound's prison fled after the missiles struck. Hamas police nabbed some of them.
One prisoner trapped under the rubble waved his hand in the hope of being rescued. Two other prisoners helped a bleeding friend walk through the debris.
Minutes after the strike, Hamas police defiantly planted the movement's green flag in the rubble.
"These strikes fuel our popular support, our military power and the firmness of our positions," said Mushir al-Masri, a Hamas legislator. "We will survive, we will move forward, we will not surrender, we will not be shaken."
Senior Hamas leaders went into hiding before the offensive began, shutting off their phones.
Hamas' Gaza prime minister, Ismail Haniyeh, spoke on a televised address on Saturday evening, but it was not immediately clear where the address was taped.
Earlier, Palestinians said Israeli bombs destroyed a mosque outside Gaza's main hospital in Gaza City; the military called it a "base for terrorist activities."
In southern Gaza, aircraft targeted a tanker truck, touching off a blaze that raged out of control and spread to about a dozen nearby houses. One of the main medicine warehouses supplying local pharmacies in southern Gaza was attacked in another sortie.
Local residents said the tanker and the warehouse contained supplies that had been smuggled in from Gaza through underground tunnels with Egypt, suggesting Israel was widening its offensive to go after businesses that are a source of income for Hamas.
Warplanes attacked the headquarters of the local Hamas television station early Sunday, but it continued to broadcast from a mobile unit.
The initial waves of attacks Saturday focused on key Hamas security installations and rocket-launching pads.
Gaza health official Dr. Moaiya Hassanain said at least 280 people were killed, including 183 members of Hamas' uniformed security forces. It was not clear how many of the others were gunmen or civilians.
The civilian casualties included a 15-year-old boy who died in southern Gaza on Sunday in an attack on a greenhouse near the border. At least 644 people were wounded, Hassanain said.
Israeli Prime Minister Ehud Olmert said it was unclear when the operation would end. The situation in southern Israel "is liable to last longer than we are able to foresee at this time," he told his Cabinet.
Shortly after, the Cabinet authorized the mobilization of 6,500 soldiers, a government official said. But the callup was not the kind of massive mobilization that was ordered in the summer of 2006 when Israel went to war against Lebanese guerrillas.
Israeli military spokesman Avi Benayahu said Israel's objective was not to halt all rocket attacks, but to cripple militants' intention and motivation to assault Israel. "To change the situation, we don't have to go after the last of the rocket launchers," Benayahu told Army Radio.
The rockets that struck close to Ashdod, extending the militants' reach closer to Israel's heartland, landed some 23 miles (38 kilometers) from Gaza. Gaza's Hamas rulers have been stockpiling weapons in recent months, including medium-range missiles. Until Sunday, the deepest targets inside Israel had been the city of Ashkelon and the town of Netivot, which are about 12 miles (20 kilometers) from Gaza.
Since the campaign began, around 150 rockets and mortars have bombarded southern Israel, according to the military's count.
Rockets repeatedly struck Ashkelon, a city of 120,000 people about 11 miles (17 kilometers) from Gaza,
Store clerk Elvira Taberbobsky, 36, stepped outside after one rocket struck only to have a second exploded right in front of her.
"I flew backwards. I couldn't hear anything for a few seconds. and then all of a sudden I saw holes in my pants and blood streaming down my pants," she said.
Schools in Israeli communities in rocket range were told to remain closed beyond the Hanukkah holiday which ends Monday.
The Palestine Liberation Organization, dominated by Abbas' Fatah movement, called a one-day commercial strike through the West Bank and urged Palestinians to take to the streets in peaceful protests.
Israel's military was on alert for possible disturbances in the West Bank. The campaign has inflamed public opinion across the Arab world, which has responded with protests and condemnations.
Crowds of thousands swept into the streets of cities around the Middle East to shout down Israel's air assault. From Lebanon to Iran, Israel's adversaries used the weekend assault to marshal crowds out onto the streets for noisy demonstrations.
In Iraq, a suicide bomber on a bicycle blew himself up in a crowd of about 1,300 protesters, killing one and wounding 16, Iraqi police said.
Hundreds gathered in the West Bank town of Ramallah, seat of Abbas' government, where demonstrators were uncharacteristically allowed to march with Hamas' green flags. "With our blood and souls we defend you, Gaza," some of the protesters chanted.
Smoke billows from the Gaza Strip following Israeli air strikes, as seen from the northern Israeli border with the Palestinian territory.
Palestinians inspect a destroyed Hamas police station following an Israeli air strike in Gaza City.
A crowd of men gather around bodies of Palestinians laid out on the ground outside Shifa hospital in Gaza City, Gaza. 
Smoke rises after an Israeli missile exploded in Rafah in the northern Gaza Strip.
A wounded child awaits medical attention at the Shifa hospital in Gaza City, Gaza.
