The flotilla set sail two weeks ago, with a total of 58 vessels joining from Spain, France and Italy aiming to break Israel’s blockade of Gaza.
On Thursday, the GSF said that, as of 04:30 GMT, at least 22 of them had been “stormed by Israeli forces in complete violation of international law”.
An earlier statement said Israeli naval forces had “intercepted vessels, jammed communications, including distress channels, and aggressively abducted civilians”.
“This is piracy. This is the unlawful seizure of human beings on the open sea near Crete, an assertion that Israel can operate with total impunity, far beyond its own borders, with no consequences,” the GSF said.
Israeli reports said the navy warned vessels to retreat and took over those which did not.
The Israeli foreign ministry also posted a video that it said showed the detained activists “making their way peacefully to Israel” aboard Israeli vessels.
Israel insists that its actions comply with international law.
On Wednesday, the Israeli foreign ministry said the flotilla was “nothing but a PR stunt: a provocation without humanitarian aid”.
“The driving force behind the flotilla provocation is Hamas – joining hands with professional provocateurs – with the aim of sabotaging President Trump’s [Gaza] peace plan transition to its second phase and intended to divert attention from Hamas’s refusal to disarm,” it alleged.
The Israeli military stopped the previous flotilla set up by the GSF from reaching Gaza last October, arresting and then deporting more than 470 people who were on board, including Swedish climate campaigner Greta Thunberg.