Most of the ships that activists wanted to sail to Gaza to break Israel's blockade have been kept in Greek ports. Greek officials have refused permission for the vessels to sail.
But one ship, the Tahrir, sailed 15 minutes out of the harbor in Crete before it was intercepted by coast guards, the activists told al-Jazeera Monday.
An al-Jazeera report said the ship, with activists from Canada, Belgium, Italy, Switzerland and Turkey, was forced to turn around and sail back to the Aghios Nikolaos port in Crete.
A coast guard ship pursued the Tahrir, using water cannons and eventually boarding the ship, an activist said.
Earlier, flotilla organizers vowed they'd try Monday to defy Greece's order stopping the ships from leaving for the Gaza Strip.
"The remaining boats are going to try to move -- the remaining boats that can move," David Heap, part of the steering committee of the Tahrir, a Canadian ship, told delegates in a Sunday briefing aboard the vessel.
"We will move [Monday]," the Toronto Star quoted Heap, 45, a University of Western Ontario professor, as saying.
Eight activists from the U.S. ship the Audacity of Hope started a hunger strike Sunday and others protested outside the U.S. Embassy in Athens in the hope of pressuring officials "to publicly acknowledge our right to sail and to call on the Greek government to free our ship and its captain immediately," U.S. activist and would-be flotilla passenger Kathy Kelly said in the statement.
Greece Friday prohibited the ships from leaving and later arrested the Audacity of Hope's 60-year-old captain after the ship tried to leave Greece without permission.
Greek Foreign Minister Stavros Lambrinidis said in light of Israel's opposition to the flotilla, there would be a "humanitarian disaster" if the vessels came under fire from the Israeli navy.
The White House has told the U.S. activists not to participate in the flotilla, saying they might violate U.S. laws because Gaza is run by the militant Hamas group, a U.S.-designated foreign terrorist organization.
Greece earlier Sunday offered to ferry the humanitarian aid to the Palestinian Authority on its navy ships, but flotilla organizers rejected the offer.