Barcelona, Spain - A flotilla carrying humanitarian aid and activists, including Swedish climate campaigner Greta Thunberg, left Barcelona on Sunday vowing to try to break Israel's siege of Gaza, organizers said.

Some 20 vessels set off from the port city on Spain's east coast just after 3.30 PM local time (9:30 AM ET) pledging to "open a humanitarian corridor and end the ongoing genocide of the Palestinian people", said the Global Sumud Flotilla.

The group defines itself on its website as an independent organization with no affiliation to any government or political party.

The flotilla, flying Palestinian flags, has hundreds of people aboard, among them activists from dozens of countries including the Irish Game of Thrones star Liam Cunningham and Spain's Eduard Fernandez.

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Also aboard were European lawmakers and public figures including former Barcelona mayor Ada Colau.

The flotilla is expected to arrive at the devastated coastal enclave in mid-September.

"The question here today is not why we are sailing. This story is not at all about the mission that we are about to embark upon," Thunberg told reporters.

"The story here is about Palestine. The story here is how people are being deliberately deprived of the very basic means to survive. The story here is how the world can be silent."

For Cunningham, "the fact that you guys are here, and the flotilla is happening, is an indication of the world's failure to uphold international law and humanitarian law, and it is a shameful, shameful period in the history of our world. And we should be collectively ashamed."

Organizers said that dozens of other vessels are expected to leave Tunisian and other Mediterranean ports on September 4 to join the aid mission.

Activists will also stage simultaneous demonstrations and other protests in 44 countries "in solidarity with the Palestinian people", Thunberg, who is part of the flotilla's steering committee, wrote on Instagram.

"This will be the largest solidarity mission in history, with more people and more boats than all previous attempts combined," Brazilian activist Thiago Avila told journalists in Barcelona last week.

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"We understand that this is a legal mission under international law," Portuguese lawmaker Mariana Mortagua, who will join the mission, told journalists in Lisbon last week.

Israel has already illegally blocked two attempts by activists to deliver aid by ship to Gaza, in June and July.

In June, 12 activists on board the sailboat Madleen, from France, Germany, Brazil, Turkey, Sweden, Spain and the Netherlands were intercepted by Israeli forces 115 miles west of Gaza. Its passengers, who included Thunberg, were abducted, detained in Israel, and eventually expelled.

In July, 21 activists from 10 countries were intercepted as they tried to approach Gaza in another vessel, the Handala. Amazon Labor Union organizer Chris Smalls, who was on the boat, was singled out for particularly violent treatment.

Israel's destruction of Gaza – facilitated by successive US regimes – has plumbed new depths in recent weeks, with a UN-backed body declaring a state of famine in the territory, warning that 500,000 people face "catastrophic" conditions.

A large and ever-growing number of human rights organizations, scholars, and legal experts have determined Israel is committing genocide against Palestinians.