More than 10 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.
Israel says its soldiers were shot at and attacked with weapons; the activists say Israeli troops came on board shooting.
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.