Greece's coastguard intercepted a boat carrying US activists within its territorial waters on Friday which was on its way to join a pro-Palestinian flotilla to Gaza, activists said.
Greek authorities have issued a sailing ban, saying that all ships with Greek and foreign flags bound for Gaza are prohibited from leaving Greek ports, DPA reported.
Activist Vangelis Pissias said the US boat named The Audacity of Hope had just left the Perama port, near Athens, when it was stopped by the Hellenic Coast Guard.
He said activists taking part in the international 10-ship flotilla will still try to leave Greek ports to break Israel's blockade of the Gaza Strip despite repeated delays that activists blame on Israeli sabotage.
"It is very sad that Greece is turning into a second Gaza," he told journalists.
The activists have rejected Israeli claims that they are carrying dangerous materials or that they planned to use violence against Israeli soldiers trying to stop them out at sea.
Minister of Citizen Protection Christos Papoutsis said that, following the orders of the Hellenic Coast Guard, all appropriate measures were being taken to implement the decision.
He said that the eastern Mediterranean would be monitored continuously to track the movement of any ships allegedly participating in the campaign.
The Ministry of Merchant Marine said that it was issuing the sailing ban under Article 128 of Greece's Sea Law which states that in cases of war ships are forbidden to leave the country's ports if it is believed that it could lead to international complications for Greece.
Israel said Monday it was "determined" to stop the flotilla from breaking its Gaza blockade, but hoped to do so without casualties, in contrast to a previous attempt last year, which left nine activists dead.
Several hundred pro-Palestinian activists from Greece, Canada, the United States and other countries plan to board the 10 ships - including two cargo vessels - carrying almost 3,000 tons of aid, medicine, a fully equipped ambulance and cement to Gaza.
Greek authorities were under pressure from Israel and other countries to delay the departure of several vessels, flotilla organizers told a news conference in Athens on Monday.
The estimated 400 passengers include at least 15 journalists and US author Alice Walker. The Israeli government dropped an earlier threat to issue deportation orders against participating journalists.
On May 31, 2010, Israel's interception of the Freedom Flotilla I turned violent when naval commandos clashed with activists wielding knives and iron bars.
The Gaza Strip-based Committee to Lift the Gaza Siege said Tuesday it was ready to receive this year's flotilla, and warned Israel not to try to intercept the ships or harm their crews.
The committee called on the United Nations to protect the ships form what it called "Israeli piracy and terrorism."