The Swedish-based Ship to Gaza rights group on Monday expressed "grief and dismay" at Greece's decision to block a humanitarian flotilla from sailing to the Palestinian territory.

In an open letter to Greek Prime Minister George Papandreaou, the group said it was "with grief and dismay that we ... received the decision of your government to block our ships from leaving Greek ports to go to Gaza."

"We are aware of the pressure that your country has been under in recent days, and feel sadness for the Greek people and the difficulties that you are enduring at this particular moment," the statement said, noting the country's efforts to avoid defaulting on its loans.

"Our mission is completely peaceful," the group added.

The organisation had planned to send a ship as part of the 2011 international "Freedom Flotilla," which Greek officials blocked from departing following international pressure, notably from Israel.

Ship to Gaza-Sweden said the flotilla was merely carrying basic goods needed for every day life and posed no threat.

On Monday morning, pro-Palestinian activists on a French boat protested the Greek decision by symbolically "setting sail" on the flotilla for Gaza, revving up their engines in an Athens port in defiance of the ban.

French activists were joined by Americans from the US Audacity of Hope boat, keen to show support after their own attempt to sail was thwarted on Friday by authorities imposing a ban on all Gaza-bound ships leaving Greek ports.

Greece has tried to appease furious activists, offering to deliver aid "through existing channels" and reaching out to the Palestinian Authority.