The Greek authorities have arrested the captain of a boat that was due to carry activists to the Gaza Strip.
John Klusmire, a US citizen, is being held in custody at police headquarters in the port of Piraeus, near Athens.
He faces charges of trying to leave port without permission and of endangering the lives of passengers.
His vessel, the Audacity of Hope, was part of a flotilla planning to take humanitarian aid to Gaza in order to challenge the Israeli blockade.
It was prevented from setting sail from the port of Perama on Friday night by the Greek coastguard, in accordance with a ban announced the same day which the Greek government said was intended to protect activists.
The Audacity of Hope, which is currently moored at a naval base, was carrying 36 passengers, four crew and about 10 members of the media.
A spokeswoman for the boat, Jane Hirschmann, told the Associated Press that the conditions of Mr Klusmire's detention were "terrible".
"There is no bed. He is sitting on a bench," she added.
The Israeli government has meanwhile denied claims it sabotaged two ships docked in Turkey and Greece which were to join the flotilla.
Israeli Foreign Ministry spokesman Yigal Palmor dismissed the accusations as "ridiculous," calling them "sad conspiracy theories".
The Turkish authorities have also said there is no evidence that the Irish vessel docked at the Aegean port of Gocek was sabotaged.
Nine activists on a Turkish aid ship were killed last year in a raid by Israeli commandos as it tried to reach Gaza.
Israel and Egypt have imposed a blockade on the coastal territory since the Islamist militant group, Hamas, seized control of it in 2007.
The Quartet of Middle East peace mediators - the UN, US, EU and Russia - said on Saturday that it remained concerned about the unsustainable conditions facing Palestinian civilians in Gaza, but noted "a marked increase in the range and scope of goods and materials" allowed in.
"The Quartet strongly urges all those wishing to deliver goods to the people of Gaza to do so through established channels so that their cargo can be inspected and transferred via established land crossings."
"The Quartet regrets the injury and deaths caused by the 2010 flotilla, urges restraint and calls on all governments concerned to use their influence to discourage additional flotillas, which risk the safety of their participants and carry the potential for escalation," it added.