"They used electric shocker devices to the extent of what we call 'electro torture' on some of the activists," lawyer Gaby Lasky told AFP, adding that at least one of them is an MP.

It was not immediately clear how many of the 30 activists and crew on board the Estelle were allegedly shot with the stun guns, which use an electrical charge to incapacitate a person.

But Israeli army spokeswoman Avital Leibovich denied that the soldiers had used "violence" in intercepting the vessel on Saturday.

"No force was used when taking over the ship," she told AFP.

There were five parliamentarians from Europe on board: Ricardo Sixto Iglesias from Spain, Sven Britton from Sweden, Aksel Hagen of Norway and Vangelis Diamandopoulos and Dimitris Kodelas from Greece.

Former Canadian lawmaker Jim Manly, who is in his late 70s, was also on board.

After leading the Estelle to the southern port in Ashdod, Israel had arrested three of its nationals who were on board for incitement. A judge extended their remand on Sunday by three days.