"I urge all my friends, family and comrades to put pressure on the Swedish government to release me and the others as soon as possible," Thunberg said in a pre-recorded message released after the ship was halted.
Rima Hassan, a French member of the European Parliament who is of Palestinian descent, was also among the volunteers on board. She has been barred from entering Israel because of her opposition to Israeli policies toward the Palestinians.
She was among six French citizens aboard the boat. French President Emmanuel Macron asked Israel to allow them to return to France as soon as possible, his office said in a statement.