The airstrikes have killed more than 300 and were prompted by repeated rocket attacks from Gaza into southern Israel, attacks that have killed two Israelis and wounded more than a dozen.
Hamas, the Islamist group that controls Gaza, fired a new round of rockets into the Israeli towns of Ashkelon and Sderot Monday despite the Israeli attacks.
Israeli warplanes responded by bombing Hamas compounds, a mosque, a laboratory building at the Islamic University, a television station and Gaza's Interior Ministry, the first government building to be targeted after days of aiming at Hamas facilities. It also hit tunnels along the border with Egypt that Israel claims are used by smugglers to bring the rockets into Gaza from neighboring Egypt.
As the militants and Israeli forces clashes, thousands of Palestinian and Israeli civilians were in danger of being caught in the crossfire. At least 57 of the some 300 killed were civilians, the United Nations reported on Monday based on visits to hospitals and medical centers.
Aid officials added that Israeli sanctions on Gaza have crippled Palestinian efforts to care for the wounded. Neighboring Egypt opened a crossing in Rafah to allow wounded to seek medical care in Egypt and medical supplies began to flow in across the border from Libya and Saudi Arabia.
The spiral of violence comes only a few weeks since a fragile ceasefire between Hamas and Israel ended.
Hamas, which has controlled Gaza since ousting the Fatah-led Palestinian Authority forces from the region 18 months ago, threatened to continue the escalation, urging its supporters to use "all available means" to fight back against Israeli attacks, including suicide bombings.
A spokesman for Israeli Prime Minister Ehud Olmert would continue until those living in southern Israel "no longer live in terror and in fear of constant rocket barrages."
The Israeli army declared the area surrounding Gaza a "closed military zone" on Monday citing the risk to civilians of rocket fire from Hamas and ordered journalists to leave. Some analysts warned the Israeli declaration, combined with the buildup of tanks and armored personnel carriers along the border, could be the precursor to a ground invasion of the region.
On Sunday, Israeli Foreign Minister Tzipi Livni defended the attacks on "Meet the Press" saying the strikes target Hamas, not civilians, and that Israel warned civilians to avoid places where Hamas is known to operate.
"The one who needs to be condemned by the international community is Hamas," Livni said, stressing a ground operation was not off the table. "Israel is a state that implements its right to defend itself and its citizens."
The Arab leaders responded to the Israeli strikes with condemnation while many civilians took to the streets to call for a stronger reaction from its leaders.
Sheik Hassan Nasrallah, the leader of Hezbollah, the Shiite group in Lebanon, that waged a war with Israel in the summer of 2006, expressed strong support for Hamas. In Iran, supreme leader Ayatollah Ali Khamenei called on Muslims to defend the Palestinians.
"All true believers in the world of Islam and Palestinian fighters are duty-bound to defend the defenseless women and children in Gaza Strip, and those giving their lives in carrying out such a divine duty are martyrs," Khamenei said in a statement, according to Iran's official news agency.
In Texas, White House spokesman Gordon Johndroe issued a statement saying that "in order for the violence to stop, Hamas must stop firing rockets into Israel and agree to respect a sustainable and durable ceasefire."
The Bush administration has also called on Israel to do everything it can to minimize the civilian toll from their military actions.