The Madleen departed from Italy on June 1 aiming to bring awareness to food shortages in Gaza, which the United Nations has called the "hungriest place on Earth". After 21 months of war, the UN has warned the territory's entire population is at risk of famine.
AFP lost contact with the Madleen early Monday morning.
At around 3:02 am CET (0102 GMT), Israeli forces "forcibly intercepted" the vessel in international waters as it was approaching Gaza, the Freedom Flotilla Coalition said in a statement.
"If you see this video, we have been intercepted and kidnapped in international waters," Thunberg said in a pre-recorded video shared by the coalition.
The Freedom Flotilla Coalition was founded in 2010 to oppose this blockade and deliver humanitarian aid.
The Palestinian resistance group Hamas condemned the diversion, saying in a statement that the boat was being taken to the Israeli port of Ashdod.
The Israeli government had vowed to prevent the "unauthorised" ship from breaching the naval blockade of Gaza, urging it to turn back.
On Sunday, Defence Minister Israel Katz said the blockade, in place since years before the Israel war on Gaza, was needed to prevent Palestinian militants from importing weapons.
After diverting the boat, Israel's foreign ministry posted a picture of the activists all in orange life jackets being offered water and sandwiches.
"All the passengers of the 'selfie yacht' are safe and unharmed," the ministry wrote on social media, adding that it expected the activists to return to their home countries.
"The tiny amount of aid that was on the yacht and not consumed by the 'celebrities' will be transferred to Gaza through real humanitarian channels," it added.
Israel is facing mounting international pressure to allow more aid into Gaza to alleviate widespread shortages of food and basic supplies.
It recently allowed humanitarian deliveries to resume after barring them for more than two months and began working with the newly formed, US-backed Gaza Humanitarian Foundation (GHF).
But humanitarian agencies have criticised the GHF and the United Nations refuses to work with it, citing concerns over its practices and neutrality.
Dozens of people have been killed near GHF distribution points since late May, according to Gaza's civil defence agency.
It said Israeli attacks killed at least 10 people on Sunday, including five civilians hit by gunfire near an aid distribution centre.
The Madleen was previously targeted on 2 May, when its generator was reportedly damaged in an Israeli attempt to disable the ship.
Rights groups have warned that any attack on the vessel would constitute a violation of international law. They compared the situation to the 2010 Mavi Marmara incident, when Israeli forces stormed a Gaza-bound flotilla, killing 10 Turkish activists.
Over 1.1 million people have signed an open letter warning Israel that any harm to the passengers or the ship will be treated as a serious violation of international law and potentially an international crime.
Activists have launched an online campaign under the slogan “We need you now to protect the flotilla and allow it to reach Gaza,” urging the public to contact their governments and demand immediate diplomatic protection for those on board.
They specifically called on the Spanish government to protect Sergio Toribio and the German government to protect Yasmin Akar.
The Freedom Flotilla Coalition also called on French President Emmanuel Macron to take action and ensure safe passage for French citizens aboard the vessel.
The group also appealed to France to uphold its obligations under the UN Charter, the Rome Statute, and the international conventions against apartheid and genocide.
'Risked their lives' for food
Civil defence spokesman Mahmud Bassal and witnesses said the civilians had been heading to a site west of Rafah, in southern Gaza, run by the US-backed Gaza Humanitarian Foundation.
Witness Abdallah Nour al-Din told AFP that "people started gathering in the Al-Alam area of Rafah" in the early morning.
"After about an hour and a half, hundreds moved toward the site and the army opened fire," he said.
The Israeli military said it fired on people who "continued advancing in a way that endangered the soldiers" despite warnings.
The GHF said in a statement there had been no incidents "at any of our three sites" on Sunday.
Outside Nasser Hospital, where the emergency workers brought the casualties, AFPTV footage showed mourners crying over blood-stained body bags.
"I can't see you like this," said Lin al-Daghma by her father's body.
She spoke of the struggle to access food aid after the two-months Israeli blockade, despite the recent easing.
At a charity kitchen in Gaza City, displaced Palestinian Umm Ghassan told AFP she had been unable to collect aid from a GHF site "because there were so many people, and there was a lot of shooting. I was afraid to go in, but there were people who risked their lives for their children and families".
The Palestinian health ministry in Gaza says at least 54,880 people, the majority civilians, have been killed in the territory since the start of the war.
Short link: