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.
