A flotilla of aid ships seeking to break Israel's blockade of Gaza has been halted by Greek authorities Reuters

Activists seeking to break Israel's blockade of Gaza with a flotilla of aid ships appeared close to defeat on Tuesday as a self-imposed deadline expired and many of the international campaigners due to sail began to return home.

Although some vowed to continue with their quest, no new date has been set for departure of the flotilla, which was supposed to be taking medicines, food, gifts and building materials to Gaza.

An Israeli law centre claimed credit for ending the flotilla's ambitions.

Just over a year after nine people were killed when Israeli marines stormed a pro-Palestinian flotilla, authorities last week banned ships destined for Gaza from leaving Greek ports, aiming to stop the latest flotilla "for their safety".

The Greeks have intercepted several of the flotilla's 10 ships as they tried to leave port in recent days, while others were forced to withdraw from the voyage due to damage which passengers blamed on Israeli sabotage.

One small French craft did manage to evade the Greek coastguard and reach international waters on Tuesday, but those on board decided not to try for Gazan waters alone and have now turned back. Meanwhile, the American captain of The Audacity of Hope, a flotilla vessel which was forced back to shore after attempting to break free on Friday, was released from custody on Tuesday. John Klusmire had been arrested on charges of setting sail without permission and endangering passengers, prompting a hunger strike from activists on board. Other protests by flotilla campaigners in Athens – including the occupation of the Spanish embassy – are ongoing.

"We will wait no matter how long it takes," Alejandro Fierro, an activist on the Spanish ship Guernica, which is docked in Crete, told al-Jazeera. "We've learned patience from the Palestinian people who have been resisting Israeli occupation for 60 years, so we can wait. We are not going to move until our government makes some solution for the Greek government to let us sail."

The Greek foreign ministry has defended its decision to stop the flotilla leaving its ports. "It is time to step up so that we can show everyone that at a critical turn of affairs for the Middle East, which is currently in the midst of tensions, there will not be developments that will exacerbate the climate further," said a ministry spokesman.

Greece and Israel completed a fortnight of joint air force drills this week, more evidence of growing ties between the two.

Meanwhile, an Israeli law centre has said it was responsible for ending the flotilla's plans. Shurat HaDin, a group with the stated objective of stemming the flow of money to terrorists, said it had been working for months to obstruct the flotilla by threatening legal action against any group that helped them in the US, Europe and Asia. "We attacked them from many different directions. It was hard work but it succeeded," said Nitsana Darshan-Leitner, the group's founder.

Even before the flotilla began to assemble, Shurat HaDin wrote to insurance and marine service companies claiming they could be aiding and abetting terrorism if they provided insurance to flotilla ships.

"Shurat HaDin was initiated to fight for Jewish rights and safeguard the state of Israel," added Darshan-Leitner. "Fighting for victims of terror was just one part of our activity. Fighting for Jewish rights, stopping the delegitimisation of Israel and preventing harm to Israeli soldiers are also important parts of our work."