The hunger striking activists are from France and Germany.
Alma Dufour, a member of La France Insoumise (LFI) party, said her colleagues Marie Mesmeur, François Piquemal, Rima Hassan, and Emma Fourreau are being "illegally detained" by Israel.
"They have begun a hunger strike to demand the release of the kidnapped crew members of the flotilla and an end to the genocide in Gaza," she wrote on the US social media platform X.
Dufour called on French President Emmanuel Macron and Foreign Affairs Minister Jean-Noel Barrot to take action in support of the detained participants.
In Belgium, Alexis Deswaef, vice-president of the International Federation for Human Rights (FIDH), and Latifa Gharbaoui, a Brussels-based activist, have also started a hunger strike after being arrested by Israeli authorities, Flotilla Belgium said, according to Belga news agency.
Deswaef and Gharbaoui were among seven Belgians participating in the humanitarian flotilla, which sought to break the blockade imposed by Israel on the Gaza Strip.
Belgium's Foreign Affairs Ministry said it was closely following the situation. "Our ambassador met personally with the Belgian flotilla participants, in particular to ensure their safety and health. The Israeli authorities intend to expel them all from their territory."
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The Global Sumud Flotilla, en route to break the Israeli blockade and deliver humanitarian aid to Gaza, approached Gaza waters on the evening of Oct. 1.
The Israeli regime’s forces attacked the flotilla, illegally seizing dozens of boats and ships and detaining hundreds of their passengers.
Source: Anadolu Agency