The Waves of Freedom Switzerland organisation said Israel had conducted “illegal and shameful attacks on the humanitarian and peaceful flotillas”, followed by “equally illegal arrests and detentions”.
“It subjected our citizens to cruel, inhumane, and degrading treatment, as defined in the International Convention against Torture, both physically and psychologically,” said the group, which goes by the acronym WOFA.
Nineteen Swiss nationals took part in the Global Sumud Flotilla, which aimed to break an Israeli blockade to deliver aid to the Gaza Strip, where the UN says famine has taken hold after two years of devastating conflict.
Israel started intercepting the flotilla vessels in international waters on Oct 1. Israeli police said more than 470 people aboard the boats were arrested.
While nine Swiss nationals have been released, 10 are still detained, WOFA said.
They were expected to be released on Tuesday, it said, adding, “We remain cautious in the face of a state that has long since abandoned any respect for the rule of law.”
Israel has denied allegations of abuse and has insisted that the legal rights of the detainees were fully respected.
But WOFA said those released had described “sleep deprivation, lack of access to water and food, lack of medical care, and verbal and psychological abuse”.
Some, it said, “were kept handcuffed for long hours without any justification”, while others “were slapped, beaten, and locked in a cage”.
In addition, “all medical treatments have been denied, including essential medicines like insulin for diabetes,” it charged.
The accusations echoed those of flotilla participants from other countries.
“The first 48 hours, there was no food, no water at all,” German flotilla member Yasemin Acar said upon her arrival in Greece on Monday.
“We were beaten. We were threatened… We were kept under the sun, our hands were cuffed to the back,” she said.
WOFA said it was “documenting the serious abuses suffered by our compatriots, as well as those they have witnessed”.
“These facts will be forwarded to the competent judicial authorities with a view to initiating legal proceedings.”