A flotilla of ships carrying humanitarian aid and activists, among whom was Swedish climate campaigner Greta Thunberg, departed from Barcelona on Sunday in what was the largest attempt yet to try to “break the illegal siege of Gaza,” organizers said.

The flotilla’s departure comes as Israel has stepped up its offensive on Gaza City and prepares to capture the densely populated city in the northern part of the enclave. As part of its preparations, Jerusalem is planning to slow or halt humanitarian aid to the area, an Israeli official told the Associated Press on Saturday.

Food experts warned earlier this month that Gaza City had fallen into famine, and that half a million people across the Strip were facing catastrophic levels of hunger — claims that were swiftly rejected by Israel.

The Global Sumud Flotilla is carrying food, water and medicine. Activists on board demanded safe passage to deliver the much-needed aid and the opening of a humanitarian sea corridor, according to a statement.

The maritime convoy departing from the Spanish port city was made up of about 20 boats and delegations from 44 countries, and was claimed to be the largest attempt to date to try and break the blockade on Gaza.

The convoy will be joined by more ships from ports in Italy and Tunisia in the coming days, on the route from the western end of the Mediterranean to the Gaza Strip, organizers said.

The flotilla will “open a humanitarian corridor and end the ongoing genocide of the Palestinian people,” said the Global Sumud Flotilla ahead of departure.

The flotilla is on course to arrive at the war-ravaged coastal enclave in mid-September.

“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.

Thousands of supporters flocked to the Barcelona pier, some of them wearing keffiyehs and chanting “Free Palestine!” and “Boycott Israel!” to send off a wide variety of boats, flying Palestinian flags, from rundown old luxury yachts to tiny wooden sailboats and industrial-looking vessels. One of them, Sirus, is more than 100 years old.

Around 70 boats are expected to take part in the final leg of the journey, flotilla spokesperson Saif Abukeshek told Spanish public television after the departure. The fleet could reach Gaza around September 14 or 15, he added.

“The story here is about Palestine. The story here is how people are being deliberately deprived of the very basic means to survive,” said Swedish activist Thunberg at a news conference. She is one of the most recognizable figures on the expedition, formed by hundreds of activists, politicians, such as the former mayor of Barcelona, Ada Colau, and journalists.

Ships carrying tons of humanitarian aid departed from the Italian city of Genoa and will join the expedition in the coming days.

It is not the first time Thunberg has attempted to reach Gaza’s waters this year. Israel deported her in June after the Madleen, the ship she was traveling on, along with 11 other people, was stopped by the Israeli military.

“It has been very clear that Israel has been continuously violating international law by either attacking, unlawfully intercepting the boats in international waters, and continuously preventing the humanitarian aid from coming in,” said Thunberg in an interview with The Associated Press on Saturday.

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

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

The Global Sumud Flotilla describes itself as an independent group not linked to any government or political party. Sumud means “perseverance” in Arabic.

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

In June, 12 activists on board the Madleen, from France, Germany, Brazil, Turkey, Sweden, Spain and the Netherlands, were intercepted by Israeli forces 185 kilometres (115 miles) west of Gaza. They were detained and eventually expelled.

In July, 21 activists from 10 countries were intercepted as they tried to approach Gaza in another vessel, the Handala.

The United Nations declared a state of famine in the territory this month, warning that 500,000 people face “catastrophic” conditions. The claim was made in a report by the Integrated Food Security Phase Classification, which Israel branded biased and inaccurate, saying it was sending enough aid into the Palestinian enclave but that Hamas steals most of it.

The war in Gaza was triggered by an unprecedented cross-border attack by the Hamas terrorist organization into Israel on October 7, 2023, which resulted in the death of some 1,200 people, mostly civilians, and the abduction of 251.

The Hamas-run Gaza health ministry says more than 62,000 people in the Strip have been killed or are presumed dead in the fighting so far, though the toll cannot be verified and does not differentiate between civilians and fighters.

Israel says it had killed over 22,000 combatants in battle as of August and another 1,600 terrorists inside Israel during the October 7 onslaught. Israel has said it seeks to minimize civilian fatalities and stresses that Hamas uses Gaza’s civilians as human shields, fighting from civilian areas including homes, hospitals, schools, and mosques.

Israel’s toll in the ground offensive against Hamas in Gaza and in military operations along the border with the Strip stands at 460.