Adalah, a rights group in Israel that said it was representing the activists, said Israel had "no legal authority" to take over the ship because it was in international waters and because it was headed not to Israel but to the "territorial waters of the state of Palestine."
"The arrest of the unarmed activists, who operated in a civilian manner to provide humanitarian aid, amounts to a serious breach of international law," Adalah said in a statement.
After a 2½-month total blockade aimed at pressuring Hamas, Israel started allowing some basic aid into Gaza last month, but humanitarian workers and experts have warned of famine unless the blockade is lifted and Israel ends its military offensive.
An attempt last month by Freedom Flotilla to reach Gaza by sea failed after another of the group’s vessels was attacked by two drones while sailing in international waters off Malta, organizers said. The group blamed Israel for the attack, which damaged the front section of the ship.