ATHENS/JERUSALEM, July 1 (Renee Maltezou and Ori Lewis/Reuters) - Greece prevented a boat of U.S. activists from sailing on Friday to join a flotilla of pro-Palestinian campaigners planning to challenge Israel's sea blockade of the Gaza Strip.

The small flotilla of activists from the United States, France and Canada was due to leave at least a week ago, but the departure has been constantly put back amid allegations of sabotage.

The Greek coastguard intercepted the boat soon after it left the port of Perama, near Piraeus, for the open sea, one of the activists said.

We are just here, two miles off Piraeus, said U.S. activist Ann Wright, aboard the ship. We've been stopped by the coastguard. Their ship maneuvered in front of us and they are now talking to the captain. They want us to turn around, she told Reuters by phone.

Some hours later the vessel started heading back to the Greek shore, said a coastguard official who declined to be named.

Wright said the mission had wanted to sail to the Greek island of Crete but the coastguard would not allow them.

Greek authorities said earlier on Friday that ships destined for Gaza -- which involves transit through international waters -- were prohibited from leaving Greek ports.

But another activist said the coast guard's interception was illegal and would not deter the rest of the flotilla, which is expected to leave in the following days.

The interception does affect us but we believe it is illegal and unreasonable, Vangelis Pissias told Reuters. We are determined to continue. How can they be doing this?

Israel says its blockade of Gaza is aimed at stopping weapons from reaching the enclave's rulers, Hamas -- an Islamist group that is branded a terrorist organization by the West.

Palestinians and their supporters say the naval blockade is illegal and constitutes collective punishment for Gaza's 1.5 million residents.

Israel has urged foreign governments not to let the latest convoy get under way. On Thursday, Israeli Prime Minister Benjamin Netanyahu thanked allies, including Greece, for taking steps to block the activists.

A spokeswoman for the campaigners said the flotilla would set sail from Greece by the middle of next week.

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 said adding that nine boats were now hoping to take part in the mission against original plans for up to 15.

Irish activists accused Israel on Thursday of sabotaging their boat in Turkey, stopping it from sailing to Gaza.

Israel has denounced the flotilla as a provocation, but has denied allegations that it sabotaged either the Irish boat or a Swedish vessel that reported propeller damage after berthing in a Greek port.

Just over a year ago, Israeli marines stormed another pro-Palestinian flotilla that had set sail from Turkey and approached Gaza. Nine activists died in the onboard clashes, provoking an international outcry.

(Additional reporting by Deborah Kyvrikosaios and Yiorgos Karahalis; Writing by Ingrid Melander, George Georgiopoulos and Ori Lewis)