Activists on board a flotilla of vessels sailing toward Gaza said the Israeli navy had intercepted at least 19 of its boats as they approached the besieged Palestinian territory.
Israeli authorities said the activists, including Greta Thunberg, were safe and being transferred to Israel.
The Global Sumud Flotilla, composed of 43 boats and 500 activists, is carrying a symbolic amount of humanitarian aid to Gaza.
The Sirius, Alma and Adara boats were intercepted some 70 nautical miles (80 miles) from the coast of Gaza, according to organisers who shared live positions of the flotilla.
The group, which also includes Nelson Mandela’s grandson, Mandla Mandela, former Barcelona mayor Ada Colau and Irish senator Chris Andrews among several European politicians, said it remained undeterred in its mission to break the Israeli blockade and bring aid to Palestinians.
Just before dawn, organisers said on their official Telegram channel that 19 of 43 boats had been intercepted and the remaining vessels would keep sailing toward Gaza.
Greg Stoker, an American veteran aboard the Ohwayla, one of the boats in the flotilla, said around a dozen naval vessels with their transponders turned off had approached it.
“They are currently hailing our vessels, telling us to turn off our engines and await further instructions or our boats will be seized and we will face the consequences,” he said in a shaky video posted on Instagram.
Israeli authorities used water cannons against some of the boats, Mr Stoker and other activists reported on social media.

The Israeli Foreign Ministry posted a video on X showing Ms Thunberg sitting on a ship’s deck being handed a water bottle and raincoat.
It said that several boats “have been safely stopped and their passengers are being transferred to an Israeli port. Greta and her friends are safe and healthy”.
Italy’s foreign minister Antonio Tajani said the boats would be towed to Israel’s port of Ashdod and the activists would be deported in the coming days.

He also said Israeli forces have been told “not to use violence”.
The flotilla, which began its journey from the Spanish port of Barcelona a month ago, had planned to reach the shores of Gaza by Thursday morning.
Activists had acknowledged that scenario was unlikely and were expecting Israeli authorities to try to stop them at any moment, as they have done in past attempts.
Israel’s government has accused some of the flotilla members of being linked to Hamas, while providing little evidence to support the claim. Activists have strongly rejected the accusations and said Israel was trying to justify potential attacks on them.
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