(Reuters) - Israeli commandos stormed a convoy of Gaza-bound aid ships on Monday and more than 10 of the mostly international activists aboard were killed, provoking a diplomatic crisis and Palestinian charges of a "massacre."

The violent end to a Turkish-backed attempt to break Israel's blockade of the Gaza Strip by six ships carrying some 600 people and 10,000 tonnes of supplies raised an outcry across the Middle East and far beyond.

As the navy escorted the vessels into Israel's port of Ashdod, accounts remained sketchy of the pre-dawn interception out in the Mediterranean, in which marines stormed aboard from dinghies and rappelled down from helicopters. Israel said "more than 10" activists died. Israeli media spoke of up to 19 dead.

The bloodshed sparked street protests and government ire in Turkey, long Israel's lone Muslim ally in the region, which had supported the convoy. Ankara recalled its ambassador from Israel and Turkish President Abdullah Gul demanded that the culprits be punished.

The European Union demanded an inquiry and France and Germany said they were "shocked." The United Nations condemned violence against civilians in international waters.

Israeli officials said the marines were met with gunfire and knives when they boarded the ships, which included a large ferry flying the Turkish flag. Activists seized at least two pistols from the boarding party, the officials said.

Israel's attempts to maintain its three-year-old blockade on the Hamas Islamist-ruled enclave while avoiding bloodshed that would spark an international incident collapsed in spectacular fashion: "It's going to be a big scandal, no doubt about it," Israel's Trade Minister Binyamin Ben-Eliezer told Reuters.

Palestinian President Mahmoud Abbas said: "What Israel has committed on board the Freedom Flotilla was a massacre." He declared three days of official mourning for the dead.

Israel's deputy foreign minister, Danny Ayalon, blamed the activists for the violence and branded them allies of Israel's Islamist enemies Hamas and al Qaeda. Had they got through, he said, they would have opened an arms smuggling route to Gaza.

There was no question of easing the blockade, he said.

In a statement, the Israeli military said that in addition to the dead, numerous activists and five soldiers were injured.

Israeli signal jamming and military censorship prevented much independent reporting of the drama at sea.

Turkish television aired video apparently showing a commando shinning down a rope and clashing with a man wielding a stick.

Israeli television showed video of an activist apparently trying to stab a soldier.

HIGH ALERT, PEACE TALKS DOUBT

Israeli forces were on high alert on the Gaza, Syrian and Lebanese borders as well as around Jerusalem, the occupied West Bank and areas of northern Israel where much of the country's Arab population lives. Israeli officials denied reports that a leading Israeli Arab Islamist had been killed on the convoy.

Angry Palestinians gathered in Ramallah, their West Bank center, and near a checkpoint to Jerusalem, which Israel closed.

Prime Minister Benjamin Netanyahu was in Ottawa. Officials said he was considering whether to cancel a White House meeting on Tuesday with U.S. President Barack Obama and fly home early.

Those talks had been expected to focus on U.S. efforts to move along tentative negotiations with Abbas. But peace talks, mediated by Obama's envoy, seem unlikely to continue for now.

Israel's Arab enemy Syria, which hosts exiled leaders of the Hamas movement that rules Gaza, called for an emergency Arab League meeting. The Cairo-based League condemned what it called Israel's "terrorist act." Iranian President Mahmoud Ahmadinejad called it "inhuman" and evidence of the Jewish state's weakness.

More worryingly for Israel, its allies were unlikely to show much sympathy. The Turkish government, long Israel's lone friend in the Muslim Middle East, "strongly protested." It marked a new low in an already crumbling Israeli relationship with Ankara.

"Israel will have to suffer the consequences of this behavior," a Turkish Foreign Ministry statement said.

Some 300 demonstrators chanted anti-Israeli slogans outside the Jewish state's Istanbul consulate. Police kept them at bay. The Israeli government advised Israeli tourists in Turkey to stay in their hotels.

Greece, which had citizens aboard the convoy, halted a joint naval exercise with Israel and summoned the Israeli ambassador in Athens. Ireland, with citizens also engaged in the venture, said it was "gravely concerned."

U.N. officials responsible for aid in Gaza said: "We are shocked by reports of killings and injuries of people on board boats carrying supplies for Gaza, apparently in international waters. We condemn the violence and call for it to stop."

"Such tragedies are entirely avoidable if Israel heeds the repeated calls of the international community to end its counterproductive and unacceptable blockade of Gaza."

DEFIANCE, AID REQUESTS

The convoy set off from international waters near Cyprus on Sunday in defiance of warnings that it would be intercepted. Israel had hoped to end the operation without bloodshed and had prepared air-conditioned tents at Ashdod for detainees.

Netanyahu's spokesman Mark Regev said: "We made repeated offers that they should bring the boats to the port of Ashdod and from there we guaranteed that all humanitarian cargo would be transferred to the people of Gaza."

Greta Berlin, a spokeswoman for the Free Gaza Movement that organized the convoy, said: "How could the Israeli military attack civilians like this? Do they think that because they can attack Palestinians indiscriminately they can attack anyone?"

Israel's Western allies have been critical of the embargo on the 1.5 million people of Gaza, which the Jewish state says is aimed at preventing arms supplies from reaching Hamas.

Turkey and Arab states were highly critical of Israel's attack on Gaza 18 months ago, in which 1,400 Palestinians died.

The United Nations and Western powers have urged Israel to ease its restrictions to prevent a humanitarian crisis and allow for postwar reconstruction. Israel says food, medicine and medical equipment are allowed in regularly.

(Writing by Alastair Macdonald, Additional reporting by Michele Kambas in Nicosia and Tulay Karadeniz in Ankara bureau; editing by Paul Taylor)