The so-called Freedom Flotilla III -- a convoy of ships carrying pro-Palestinian activists, at least one European lawmaker and an Arab-Israeli MP -- will try to reach the shores of the Gaza Strip by the end of the month.
Their campaign comes as Israel faces heavy international pressure over its actions in Gaza, with a UN report released yesterday saying both the Jewish state and Palestinian militants may have committed war crimes during last year's conflict in the besieged coastal enclave.
"We're not alone in considering the blockade to be inhumane and illegal," Staffan Graner, an activist who is sailing aboard Swedish trawler the Marianne of Gothenburg, told AFP.
"What we want to do... Is to keep up international pressure that the blockade should end," he said.
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The Marianne of Gothenburg, which set sail from Sicily on Friday, will join four other vessels carrying some 70 people en route for Gaza, according to a statement from the Platform of French NGOs for Palestine, an advocacy group supporting the effort.
Ghattas's decision to join the flotilla caused outrage in Israel.
On Sunday, deputy foreign minister Tzipi Hotovely said the flotilla was "the work of provocateurs whose aim is to blacken Israel's face," adding that the ministry had been working "through diplomatic channels night and day" to prevent it from reaching Israeli waters.
In a letter to Prime Minister Benjamin Netanyahu, Ghattas urged that he "command the Israeli security forces to stay away and allow the flotilla to arrive at its destination."
Ghattas was referring to the killing of 10 Turkish activists aboard the Mavi Marmara after Israeli commandos staged a botched pre-dawn raid on a six-ship flotilla seeking to break the Gaza blockade in May 2010.