A second flotilla carrying humanitarian aid to Palestinians in Gaza sailed from the Spanish port of Barcelona on April 12, aiming to try to break the Israeli blockade.
The vessels were seized by Israel hundreds of miles from Gaza, said the organisers, Global Sumud Flotilla.
"This is piracy," the group said in a statement.
"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."
No state had the right to claim, police, or occupy international waters, but Israel had done that, extending its control outward to occupy the Mediterranean Sea off the coast of Europe, it added.
Israel's UN envoy Danny Danon said the flotilla "was stopped before reaching our area".
In a posting on X, he added, "Our brave IDF soldiers are acting with professionalism and determination dealing with a group of delusional attention-seeking agitators," referring to the Israeli military.
Israel's military halted a previous flotilla assembled by the same organisation last October in an attempt to reach blockaded Gaza, arresting Swedish activist Greta Thunberg and more than 450 participants.
Israel, which controls all access to the Gaza Strip, denies withholding supplies for its residents, numbering more than two million.
Yet Palestinians and international aid bodies say supplies reaching the territory are still insufficient, despite a ceasefire reached in October that included guarantees of increased aid.
with DPA
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