"If you see this video we have been intercepted and kidnapped in international waters," she said.
In another pre-recorded message, Brazilian activist Thiago Ávila called for support for him and the other members of the flotilla who have been detained.
Freedom Flotilla organiser Huwaida Arraf said Israel did not have any authority to detain the volunteers who were aboard the ship.
"Their detention is arbitrary, unlawful, and must end immediately," she said.