Here are some of the key reactions.
EU
"No one should be punished for defending humanity," said European Commissioner Hadja Lahbib.
"Watch this video. These are not convicted criminals. These are activists trying to get bread to the hungry," Lahbib, whose brief is equality, preparedness and crisis management, wrote on X.
Italy
Italian Prime Minister Giorgia Meloni called for the immediate release of detained Italian activists and demanded an apology from Israel.
"It is intolerable that these protesters, among whom there are many Italian citizens, are subjected to this treatment, which violates human dignity," Meloni said in a statement.
France
France summoned Israel's ambassador to protest at the actions of Israeli National Security Minister Itamar Ben Gvir, who posted the footage.
"I have requested that the Israeli ambassador to France be summoned to express our indignation and obtain an explanation," French Foreign Minister Jean-Noel Barrot wrote on X.
Ireland
Irish Foreign Minister Helen McEntee said she was "appalled and shocked" by the video and demanded the immediate release of the "illegally detained" activists.
Among the detainees is the sister of Irish President Catherine Connolly.
Spain
Spanish Foreign Minister Jose Manuel Albares said the Israeli envoy in Madrid had been summoned.
"That treatment is monstrous, disgraceful and inhumane," Albares said.
Turkey
Turkey's foreign ministry said Ben Gvir "has once again openly demonstrated to the world the violent and barbaric mentality of the Netanyahu government".
Germany
Germany's ambassador to Israel Steffen Seibert said the treatment of the activists was "wholly unacceptable".
"It is good to hear many Israeli voices -- including the foreign minister -- call out in all clarity Minister Ben Gvir's treatment of the detainees for what it is: wholly unacceptable and incompatible with the basic values of our countries," Seibert wrote on X.
Greece
Greece's foreign ministry called for the release of Greek activists -- thought to number 19 -- to be released.
"The conduct of the Israeli Minister of National Security, which targeted citizens participating in the 'Global Sumud Flotilla,' is unacceptable and absolutely condemnable," the ministry said in a statement.
Belgium
Belgian Foreign Minister Maxime Prevot said the Israeli ambassador would be summoned to explain the video.
"Belgian citizens are among those detained. This situation is unacceptable. It violates the most basic standards of human dignity. All detainees must be treated with dignity and released without delay," Prevot wrote on X.
Netherlands
Dutch Prime Minister Rob Jetten wrote on X that the Israeli ambassador had been summoned, while Foreign Minister Tom Berendsen called the treatment of the activists "shocking and unacceptable".
Poland
Polish Foreign Minister Radoslaw Sikorski demanded "justice for our citizens" among those detained, and "consequences" for Ben Gvir.
"In the democratic world we do not abuse and gloat over people in custody," he said on X.
Canada
Canada's Foreign Minister Anita Anand said the footage was "deeply troubling and absolutely unacceptable".
"I have directed my officials to summon the Israeli ambassador regarding the mistreatment of civilians aboard the flotilla. What we've seen, including the video shared by Itamar Ben Gvir, is deeply troubling and absolutely unacceptable," Anand said.
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