ASHDOD, Israel -- A flotilla of pro-Palestinian activists bound for the Gaza Strip met deadly resistance from Israeli forces Monday, sparking a diplomatic crisis in the Middle East and endangering the Obama administration's attempt to revive peace talks between Israel and the Palestinians.

Nine people were killed and dozens more were wounded when Israeli navy commandos stormed the Marmara, a Turkish ship that was part of a six-boat flotilla aimed at breaking the Israel-led blockade of the Gaza Strip.

Israel's allies in Europe and the United Nations demanded an investigation into the violence, and a spokesman for Israeli Prime Minister Binjamin Netanyahu announced that the premier had canceled a trip to Washington that was meant to signal warming relations between Israel and the Obama administration.

In a statement, the White House said that President Barack Obama understood Netanyahu's decision to return immediately to Israel and added that the two had agreed to reschedule their meeting at the first opportunity.

"The president expressed deep regret at the loss of life in today's incident, and concern for the wounded, many of whom are being treated in Israeli hospitals," the statement said. "The president also expressed the importance of learning all the facts and circumstances around this morning's tragic events as soon as possible."

Thousands online have viewed footage of the incident, which activists aboard the ship filmed. Israeli commandos, clad in black and wearing face masks, are seen rappelling from Blackhawk helicopters onto the Marmara, where others in Zodiac rubber boats quickly joined them.

The footage then cuts to activists in orange vests carrying what appear to be wounded men across the boat. Fighting can be seen between Israeli soldiers and activists wielding slingshots and wooden batons.

Israeli armed forces chief of staff Lt. Gen. Gabi Ashkenazi said the situation aboard the Marmara, which is owned by the IHH Islamic charity, differed from that on the other five ships.

"There was extreme violence from the moment that our forces reached the ship. It was premeditated and included weapons, iron bars, knives and at a certain stage firearms, perhaps in some cases weapons that were snatched from soldiers," he said.

An eyewitness report by Israeli journalist Ron Ben Yishai described how the naval commandos were ill-prepared to meet violent resistance.

He said the Israeli commandos initially were armed with paintball guns, but later returned live fire. At least two guns were wrested away from the Israeli soldiers, the IDF said.

Israeli military representatives confirmed that eight commandos were injured during the operation, at least one of them shot with a fellow soldier's weapon. Representatives for the activists deny the account, and claim the IDF is distorting what happened.

Demonstrations were held Monday throughout the Palestinian territories, and in Egypt, Jordan, Iraq, Lebanon, Syria and Turkey.

In Cairo, more than 200 activists and parliament members raising banners and Palestinian flags gathered in front of the Egyptian foreign ministry.

"The (Egyptian) foreign ministry is a traitor and a cause for shame. Whoever normalizes relations with Israel is a traitor and a coward," demonstrators chanted.

"This is Israeli thuggery, and it is not being deterred by anyone," said Essam al-Erian, a prominent member of Egypt's largest opposition group, the Muslim Brotherhood. The group said it hasn't received news about two of its members, Mohamed al-Biltagy and Hazem Farouk, members of the Egyptian parliament who were in the aid flotilla.

Iraqi Vice President Tariq al-Hashimi, a Sunni Muslim, wrote to Turkish Prime Minister Recep Tayyip Erdogan, saying: "I was shocked at the barbaric actions committed by Israel against the Freedom fleet that was headed toward Gaza Strip. This incident is further evidence of Israel's deviation from and its disdain for international laws and mandates . ..."

More than 500 Iraqis loyal to militant Shiite cleric Muqtada al-Sadr gathered in Baghdad on Monday afternoon and burned and stomped on a U.S. flag.

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Abdulhadi al-Muhammedawi, a prominent tribal sheikh in southern Iraq known for his opposition to Saddam Hussein, told McClatchy Newspapers that all Iraqis should raise their voices in condemnation.

"This vile crime cannot be overlooked by the international community. I call on the international community to shoulder its responsibility and look into the violations of human rights that are taking place against the Palestinian people -- Israeli terrorism is just that: terrorism. Why are they silent?" he said.

Arab League Chief Amr Moussa called the raids terrorist attacks. "Israel feels it is above the law and can do anything, and is not under the authority of the (United Nations) Security Council," Moussa said in Doha, Qatar. "It is vital that we reconsider how we deal with the Arab-Israeli struggle, since what happened is proof that Israel is not ready for peace."

The U.N. Security Council scheduled an emergency meeting to discuss the violence, while the Arab League and the Palestinian Authority cabinet called emergency meetings to discuss the future of the U.S.-brokered indirect peace talks with Israel.

Netanyahu, who expressed his "full backing" for the military raid, said he hoped for support from Israel's allies and for patience as Israel investigates the events.

The flotilla of activists was headed to Gaza to draw attention to the blockade on the coastal territory, which Israel imposed after the militant Islamist group Hamas seized control of the territory in 2007.

More than 700 activists were in the flotilla, which they said carried approximately 10,000 tons of aid to Gaza.

Israel has said that the aid will be inspected and then delivered via land channels to the Gaza Strip. All the activists aboard the ship will be treated for injuries, and then undergo deportation proceedings, said Israeli officials.