Israel was condemned on Thursday by much of the Arab world while securing Western backing and pressing its biggest air assault on Gaza for years.
Iran and Egypt – whose new Islamist leaders are under pressure to build closer ties with the Palestinians at the cost of a 30-year peace deal with the Jewish state – led the angry protests against strikes that left at least 19 dead.
"The Israelis must understand that we do not accept this aggression, which can only lead to instability in the region," Egypt's President Mohamed Morsi said in televised remarks.
Iran – accused by Israel of being the Gaza militants' main supplier of rocket power – branded the Israeli strikes as "organised terrorism" conducted by "criminal ... Zionist (Israeli) military forces".
Tehran called on the United Nations and the European Union to halt the "barbaric" offensive against the Gaza Strip.
"Immediate and serious action by international [organisations] is needed to end the military campaign against the people of Gaza," foreign minister Ali Akbar Salehi said in a statement carried by the official IRNA news agency.
The call was made in separate letters to UN chief Ban Ki-moon, EU foreign policy chief Catherine Ashton and Organisation of Islamic Cooperation secretary general Ekmeleddin Ihsanoglu, IRNA reported.
And the increasingly influential Arabian Peninsula state of Qatar warned starkly that the "vicious attack [on Gaza] must not pass unpunished" and demanded urgent action at the United Nations.
Concern, calls for restraint from world leaders
An emergency UN meeting concluded Wednesday without a decision and clear signals of concern over the first escalation of Middle East violence since the 2011 Arab Spring altered the political map of the historically volatile region.
On Thursday, the United States called on Egypt, Turkey and European powers to press Hamas to end bloodshed in Gaza, putting the onus on the Islamist movement to end rocket attacks on Israel.
The White House said it was in close contact with Israel about its military campaign into the Hamas-controlled territory, in a conflict that officials said has killed 16 Palestinians and three Israelis.
British Prime Minister David Cameron told Israeli counterpart Benjamin Netanyahu he was "extremely concerned" by violence in Gaza and urged him to do all he could to avoid civilian casualties.
French President Francois Hollande meanwhile held talks with Morsi on the phone, expressing France's "deep concern" and calling for "restraint", the president's office said.
He "stressed the role Egypt could play to cut the tensions", adding: "All efforts by the international community must converge to avoid any unilateral initiative and any provocation that could worsen the situation."
Turkish foreign minister Ahmet Davutoglu called the Israeli offensive a "crime against humanity", Anatolia news agency reported, adding that he had spoken with US Secretary of State Hillary Clinton about ways to lower the tension in the region.
Russian President Vladimir Putin urged both Netanyahu and the Palestinians to avoid escalating the violence as Moscow worried about the fighting spreading to other regions of the volatile Arab world.
"The president of Russia called on the parties to exercise restraint and avoid the path of escalating violence, whose victims include civilians, and to do everything to return the situation to its normal course," the Kremlin said, following a telephone conversation between Putin and Netanyahu.
But Moscow also criticised the "disproportionate strikes on Gaza" while calling for cooler heads.
US 'supports Israel's right to defend itself'
Washington meanwhile rose to the Jewish state's defencedespite earlier signs of strains in relations between US President Barack Obama and the conservative Netanyahu.
Expressing regret for the victims on both sides of the conflict, White House spokesperson Jay Carney said there was "no justification" for the violence on the part of Hamas, saying it "does nothing to help the Palestinians".
US state department spokesperson Mark Toner said in a statement: "We support Israel's right to defend itself, and we encourage Israel to continue to take every effort to avoid civilian casualties."
The United States is also calling on Egypt to use its sway with the Palestinians to try to end the violence from Gaza, Toner said.
French Prime Minister Jean-Marc Ayrault said it was "time to end this dangerous escalation", adding that Paris was involved in "intense diplomatic activity to lower tensions".
"France will do everything to avert an escalation," he told journalists at the French embassy in Berlin where he was visiting.
The tit-for-tat attacks and prospects of a possible ground invasion of Gaza prompted Palestinian President Mahmoud Abbas to cut short a tour of Europe aimed at winning support for his attempts to upgrade his territory's UN status next month.
But much of the diplomatic attention this week has focused instead on the response to the tensions by Egypt—a powerbroker in the conflict for decades whose peace deal with Israel has come under growing pressure in recent months.
Morsi speaks out
Morsi has crafted a new vision for his country since assuming office in June that involves a mix of backing for traditional Arab causes and an effort to remain on cooperative terms with the West.
The new Egyptian leader said that he told Obama by telephone that he was not able to condone attacks that were killing innocent Palestinian civilians in one of the most densely-populated regions on earth.
Israel has also confirmed that Egypt was withdrawing its ambassador to the Jewish state in protest over the air raids.
"Shortly before dawn, I called President Obama and we discussed the need to put an end to this aggression and to ensure it does not happen again," said Morsi.
"I explained Egypt's role, Egypt's position, that we have relations with the United States and the world, but at the same time we totally reject this aggression," the Egyptian leader stressed.
Egyptian Prime Minister Hisham Qandil will visit the Gaza Strip on Friday, Gaza's Hamas government said. -– Sapa-AFP