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 vessels overnight, clashing with some of the 600 protesters on board.
The exact location of the interception is unclear. Israel had warned the ships not to enter its territorial waters.
The ships are carrying 10,000 tonnes of aid to try to break a long-standing Israeli-led blockade.
Israel says its forces were attacked by activists when they got on board.
"The people on the boats were very, very violent toward the soldiers," Israeli military spokeswoman Lt Col Avital Leibovich was quoted as saying by AFP news agency.
Organisers of the flotilla said at least 30 people were wounded in the incident.
Condemnation
An Israeli government minister, Binyamin Ben-Eliezer, was reported as expressing regret for "all the fatalities".
Turkey, one of whose charities is a main organiser of the flotilla, said it "strongly condemn[ed] these inhumane practices of Israel", AFP said.
Turkish TV pictures taken on board the Turkish ship leading the flotilla appeared to show Israeli soldiers fighting to control passengers.
The footage showed a number of people, apparently injured, lying on the ground. It is not clear whether the fighting is continuing.
Al-Jazeera TV reported from the same ship that Israeli navy forces had fired and boarded the vessel, wounding the captain.
The Al-Jazeera broadcast ended with a voice shouting in Hebrew, saying: "Everybody shut up!"
'Provocation'
The six-ship flotilla left international waters off the coast of Cyprus on Sunday and was expected to arrive in Gaza later on Monday.
Israel has said it would stop the boats, calling the campaign a "provocation intended to delegitimise Israel".
An economic blockade was imposed by Israel after the Islamist movement Hamas took power in Gaza.
Israel says it allows about 15,000 tones of humanitarian aid into Gaza every week.
But the United Nations says this is less than a quarter of what is needed.
