2:23 PM EDT UPDATE: US Boat to Gaza now detained within barbed wire lined Greek Coast Guard base. "They are letting press off the boat but passengers are kept on board"
1:22 PM EDT UPDATE: @medeabenjamin "Psngrs were serene & ready for Greek commandos to board; we were not going to move but our captain decided to go back."
1:05 PM EDT UPDATE: US Boat to Gaza leadership decides to follow orders, return to Greek port.
12:45 PM EDT UPDATE: Tweets from the US Boat to Gaza:
"Captain refuses to turn back. Passengers preparing for troop raid of ship."
"7- or 8- commandos on Greece Coast Guard boat in full gear holding m-16s."
"Greek commandos are threatening the US boat with guns pointed at passengers"
"We urge you to immediately contact the Greek Embassy in Washington, DC at 202-939-1300 and the Greek consulate in your city. Tell them to let The Audacity of Hope leave Athens safely and sail to Gaza."
11:53 AM EDT UPDATE: The Greek government has now put out this statement on the Gaza flotilla:
Prohibition of the departure of ships with greek and foreign flags from greek ports to the maritime area of Gaza
Pursuant to a decision by the Minister of Citizen Protection Mr. C. Papoutsis, the departure of ships with greek and foreign flags from greek ports to the maritime area of Gaza has been prohibited today.
By orders of the Hellenic Coast Guard Head Quarters to all local Hellenic Coast Guard Authorities, all appropriate measures are taken for the implementation of the said decision.
As it is known, the Hellenic Coast Guard local Authorities have already been instructed to submit to ships Masters of ships of any flag the relevant notice to mariners that designates the area of maritime blockade by Israel.
More specifically, for ships flying the greek flag, relevant entries shall be made in the ships logbook together with the reception of the above mentioned notice to mariners as well as with the communication by the Ministry of Foreign Affairs, dated 22 June 2011.
Furthermore, the broader maritime area of eastern Mediterranean will be continuously monitored by electronic means for tracking, where applicable, the movements of the ships allegedly participating in such campaign.
Finally, police controls of passengers and crews are conducted while there is cooperation with local police and customs Authorities for rigorous controls on issues of their competence.
10:33 AM EDT UPDATE: The Greek Coast Guard has now stopped the US boat and is demanding that they return to the Port of Athens
Organizers of the US boat participating in the flotilla to Gaza said Friday afternoon that their boat, The Audacity of Hope, had left the Greek harbor where it was docked, they said through the organization's Twitter feed.
Minutes earlier, organizers said that the ship's passengers were preparing to "non-violently resist any efforts to stop the boat," saying moments later that the boat "is about to set sail." Through the boat's twitter feed, organizers said that a Greek Coast Guard vessel approached The Audacity of Hope as it was pulling away from the dock but then sped away.
Earlier Friday, a spokesperson for the larger flotilla effort said that the ships would set sail next week after repeated delays that activists blame on Israeli sabotage.
"Israel is doing its very, very best to make sure we don't get out of port," spokeswoman Greta Berlin said, speaking by telephone from Greece where some of the ships are moored.
"We want to move the boats by July 5 to get to our rendezvous point no later than July 6 or 7 ... We will go with what we have," she added.
The small flotilla, including boats from the United States, France and Canada, was due to leave at least a week ago, but the departure has been constantly put back amidst allegations of a dirty tricks campaign to stop the pro-Palestinian initiative.
Israel says its blockade of Gaza is aimed at stopping weapons from reaching the enclave's rulers, Hamas -- an Islamist group which is branded a terrorist organization by the West.
Palestinians and their supporters say the naval closure is illegal and constitutes collective punishment for Gaza's 1.5 million residents.
Irish activists accused Israel on Thursday of sabotaging their boat in Turkey by damaging the propeller shaft, stopping it from sailing to Gaza.
Berlin said the Greek authorities had not yet allowed a U.S. boat, "The Audacity of Hope", to leave the port of Piraeus.
"The Greeks are cooperating with Israel as much as they can," she said, adding that nine boats were now hoping to take part in the mission, compared with original plans for up to 15 ships.
Israel has denounced the flotilla as a provocation, but has denied allegations it sabotaged both the Irish boat and a Swedish vessel, which also reported propeller damage after berthing in a Greek port earlier in the week.
Israel has urged foreign governments not to let the second convoy get under way. In a speech on Thursday, Israeli Prime Minister Binyamin Netanyahu thanked allies, including Greece, for taking steps to block the activists.
"I want to thank many leaders in the world for speaking and acting recently against the provocative flotilla," he said. "Israel has the full right to act against the efforts to facilitate rockets-smuggling to Hamas's terror enclave."