Protesters on a boat bound for the Gaza Strip were escorted back to shore as Greece announced it was banning vessels heading to Gaza from leaving its ports.
The secretive attempt by the American activists to head out to sea ended in failure after authorities in inflatable speedboats raced after them when their vessel tried to sail without permission from the port of Perama near Athens.
"We shall overcome," the activists sang as security personnel watched from their boat just 10 metres away, according to updates protesters posted on the internet during a brief stand-off.
Greek officials appealed to them to turn around, arguing that it was not safe to continue, but activists responded that it was not safe in port because of fears of alleged sabotage of their vessels, organisers said.
On Thursday an Irish ship, the MV Saoirse, said it had to abandon plans to set sail from the Turkish town of Gocek because of what it called Israeli sabotage. Earlier this week, activists said the propeller of a Swedish ship in a Greek port was sabotaged. Israel has not commented on the reports.
The Greek government action delivered a major blow to a flotilla of nine Greek and foreign-flagged vessels and several hundred activists who had said they want to break Israel's sea blockade and deliver aid to the Palestinian territory.
The setback followed a week of administrative delays that organisers attributed to Israeli pressure on Greece, which is mired in an economic crisis and has grown closer to Israel as it seeks more foreign investment.
Israel says its sea blockade stops weapons from reaching Iran-backed Hamas militants who control Gaza, and had warned it would stop any attempt to circumvent its restrictions. A year ago, nine activists on a Turkish boat died in an Israeli raid on a similar flotilla, with each side accusing the other of starting the violence.
In Jerusalem, Israeli military spokeswoman Lt Col Avital Leibovich claimed that according to intelligence information a son-in-law of Hamas leader Khaled Mashal, Hamoud Tareq, was among the organisers of the flotilla. She said that Tareq is active in the Islamic militant Hamas movement and other terror groups.
Greece's Civil Protection Ministry said coastguard authorities were ordered to take "all appropriate measures" to implement the ban. It also said the "broader maritime area of the eastern Mediterranean will be continuously monitored by electronic means for tracking, where applicable, the movements of the ships allegedly participating" in the flotilla.