The "Louise Michel" and its crew tried to set sail on a humanitarian mission to Gaza on Monday but were blocked by the Greek coast guard.
This is the second vessel from the Gaza Freedom Flotilla to be barred from sailing from Greece.
Last week a boat of American activists was also prevented from leaving after Athens banned ships aiming to break the blockade.
Some of this boat's 24 passengers told coast guards they should be allowed to carry out their mission.