At least 19 people have been killed after Israeli commandos stormed a convoy of ships carrying aid to the Gaza Strip, the Israeli army says.
Armed forces boarded the largest vessel overnight, clashing with some of the 500 people on board.
It happened about 40 miles (64 km) out to sea, in international waters.
The activists were attempting to defy a blockade imposed by Israel after the Islamist movement Hamas took power in Gaza in 2007.
There has been widespread condemnation of the violence, with several countries summoning the Israeli ambassadors serving there.
UN chief Ban Ki-moon said he was “shocked by reports of killings and injuries” and called for a “full investigation” into what happened.
Israeli Prime Minister Benjamin Netanyahu, who is in Canada, has cancelled a scheduled visit to Washington on Tuesday to return to Israel, officials said.
Earlier, he expressed his “full backing” for the military involved in the raid, his office said.
The White House said the US “deeply regrets the loss of life and injuries sustained” in the storming of the aid ship. The six-ship flotilla, carrying 10,000 tonnes of aid, left the coast of Cyprus on Sunday and had been due to arrive in Gaza on Monday. Israel had repeatedly said the boats would not be allowed to reach Gaza.
Israel says its soldiers boarded the lead ship in the early hours but were attacked with axes, knives, bars and at least two guns.
“Unfortunately this group were dead-set on confrontation,” Israeli government spokesman Mark Regev told the BBC.
“Live fire was used against our forces. They initiated the violence, that’s 100% clear,” he said.
Organisers of the flotilla said at least 30 people were wounded in the incident. Israel says 10 of its soldiers were injured, one seriously.
A leader of Israel’s Islamic Movement, Raed Salah, who was on board, was among those hurt.
Audrey Bomse, a spokesperson for the Free Gaza Movement, which is behind the convoy, told the BBC Israel’s actions were disproportionate.
“We were not going to pose any violent resistance. The only resistance that there might be would be passive resistance such as physically blocking the steering room, or blocking the engine room downstairs, so that they couldn’t get taken over. But that was just symbolic resistance.”
She said there was “absolutely no evidence of live fire”.
Israel is towing the boats to the port of Ashdod and says it will deport the passengers from there.
Al-Jazeera TV reported from the ship that Israeli navy forces had opened fire and boarded the vessel, wounding the captain.
Israel’s deputy Foreign Minister Danny Ayalon said his country “regrets any loss of life and did everything to avoid this outcome”.
He accused the convoy of a “premeditated and outrageous provocation”, describing the flotilla as an “armada of hate”.
Palestinian Authority President Mahmoud Abbas condemned Israel’s actions, saying it had committed a massacre, while Hamas said Israel had committed a “great crime and a huge violation of international law”.
Turkey, whose nationals comprised the majority of those on board, accused Israel of “targeting innocent civilians”.
“We strongly denounce Israel’s inhumane interception,” it said, warning of “irreparable consequences” to the two countries’ relations.
Danny Ayalon, Israeli deputy foreign minister: “The organisers’ intent was violent.”
Turkey was Israel’s closest Muslim ally but relations have deteriorated over the past few years.
In Turkey, thousands of protesters demonstrated against Israel in Istanbul, while several countries have summoned Israeli ambassadors to seek an explanation as to what happened.
Greece has withdrawn from joint military exercises with Israel in protest at the raid on the flotilla.
Israel had repeatedly said it would stop the boats, calling the campaign a “provocation intended to delegitimise Israel”.
Israeli military spokesperson Avital Leibovich said: “This happened in waters outside of Israeli territory, but we have the right to defend ourselves.”
Condemnation by Amnesty International
Meanwhile, Amnesty International has called for Israel to launch an immediate, credible and independent investigation into the killing by its armed forces on boats protesting the Israeli blockade on the Gaza Strip.
“Israeli forces appear clearly to have used excessive force,” said Malcolm Smart, Amnesty International’s director for the Middle East and North Africa.
“Israel says its forces acted in self-defence, alleging that they were attacked by protestors, but it begs credibility that the level of lethal force used by Israeli troops could have been justified. It appears to have been out of all proportion to any threat posed.”
For nearly three years, Israel, which is the occupying power in the Gaza Strip, has implemented a policy of banning all movement of goods and people, except for the most basic humanitarian necessities, which are imported by international aid agencies. Only a fraction of patients in need of treatment outside Gaza are allowed out, and dozens have died waiting for Israeli permission to travel.
“The blockade does not target armed groups but rather punishes Gaza’s entire population by restricting the entry of food, medical supplies, educational equipment and building materials,” said Malcolm Smart.
“Unsurprisingly, its impact falls most heavily on those most vulnerable among Gaza ‘s 1.5 million people: children, the elderly and the sick. The blockade constitutes collective punishment under international law and must be lifted immediately.”
Israel has a duty under international law to ensure the welfare of Gaza ‘s inhabitants, including their rights to health, education, food and adequate housing., the Amnesty stated.