Israel launched Operation Cast Lead on Saturday, destroying Hamas police stations and interior ministry buildings.
It was the deadliest day in Gaza for decades, with Israel continuing to attack sites into the night and threatening that the operation would widen if necessary.
Ehud Olmert, the outgoing Israeli prime minister, described the assault as a war on Hamas, the Palestinian faction which took control of the Gaza Strip in June 2007.
Members of the Israeli cabinet say the attack is in response to an increase in the number of home-made rockets being fired into southern Israel since a ceasefire ended on December 19.Carnage
Many of the dead were police officers, including Tawfiq Jabber, the Gaza chief of police.
The toll is expected to rise further, with bodies still lying buried under the rubble of destroyed buildings. Hospitals, already suffering from shortages due to an 18-month blockade on the Gaza Strip, said they were struggling to cope with the number of injured, which included women and children.
Gaza is densely populated. Its 1.5 million residents area already experiencing shortages in power and basic supplies due to the siege which is widely condemned by human rights movements as a collective punishment.
In an exclusive interview with Al Jazeera, Meshaal, who lives in exile in Syria, said: "I call upon you [Palestinians] to carry out a third intifada [uprising]."
He called for a "military intifada against the Zionist enemy", as well as "a peaceful intifada internally", in reference to Hamas's power struggle with Mahmoud Abbas, the Palestinian president, whose government is based in the Israeli occupied West Bank.
Ismail Haniya, the Hamas leader in Gaza, called the assault Israel's "ugliest massacre".
"I call on Palestinians to remain united and together in the face of this crime, in the face of this massacre and continued aggression, targeting our soil and our citizens," he said.
Olmert, speaking in Tel Aviv on Saturday, said the operation would take time and called on Israelis to be "patient".
"The quiet we offered was answered with mayhem. Our desire for calm was answered with terror," he said.
"You are not our enemies. We do not fight against you," Olmert said in a direct address to Palestinians.
"[Terror organisations] are disastrous for both peoples. Israel is not fighting against the Palestinian people, and the targets attacked today were chosen with the intent of avoiding civilian casualties."
Jacky Rowland, Al Jazeera's correspondent in Jerusalam, said: "People have been forewarned about further operations of this intesnsity for many days to come, with more sorties flown by Israeli planes and helicopters and more targets in Gaza.
"In response, more than 50 rockets were fired into Israel today. Defence officials are warning that there could be as many as 200 rockets fired every day into Israel in the days to come."
One Israeli was killed in rocket fire on Saturday.
Salam Fayyad, the Palestinian prime minister, condemned the attack and demanded an immediate cessation.
Many leaders added their voices to condemn the onslaught, including Ban Ki-moon, secretary-general of the UN, who called for an immediate cessation of hostilities.
Mousa Abu Morzouz, the deputy leader of Hamas, said: "Nobody in this world can accept what happened and the Israeli aggression ... [we expect] the international community to stand against this and say that it is not acceptable."
Mustafa Barghouthi, the former Palestinian information minister, said: "This is not an attack on Hamas. It is an attack on the whole population and the free will of the people of Gaza."
He accused Israel of committing "war crimes" and demanded that Abbas and his government stop all relations with Israel.
'Only just beginning'
The Israeli army released a statement on Saturday saying "terrorist installations" were hit and that all Israeli pilots returned unharmed.
Avi Benayahu, an Israeli military spokesman said: "The operation against Hamas is "only just beginning".
The air raids follow a breakdown of a six-month-old Israel-Hamas truce earlier this month.
The ceasefire expired on December 19, with Hamas arguing that Israel had violated the truce by preventing vital supplies from entering the Strip.
Egypt has opened the Rafah crossing with the Gaza Strip to receive injured people, Egyptian officials said. Ambulances have been sent to the crossing and two Egyptian hospitals emptied to take in the wounded.
Hamas won control of the Palestinian Legislative Council in elections in January 2005. The international community refused to accept a Hamas-led government, demanding that the faction recognise Israel and renounce violence. Economic sanctions by the EU and US followed.
Hamas took control of the Gaza Strip in June 2007 after bloody street battles against its rival, Fatah.
Source: Al Jazeera