ATHENS (AFP) - Jumpy activists on the ill-fated mission to Gaza insist the evidence is mounting that Israeli secret services are behind the slew of mishaps that have dogged the flotilla.

Two "sabotaged" boats and a stream of bureaucratic obstacles have prevented the convoy from setting sail from Greece to challenge the sea blockade of the Gaza Strip, and passengers have accused Israel of dirty tricks and espionage.

Israel has laughed off the allegations as "paranoid and ridiculous," mocking the pro-Palestinian activists for believing life was like "a James Bond film" and suggesting they "leave the movie and return to reality."

But activists say their operation has been plagued by ill-fortune that bears the hallmark of the Mossad secret service.

Suspicions were aroused after news that the Irish boat, Saoirse, had been "sabotaged in a dangerous manner" in the Turkish port of Gocek, just days after the Swedish-owned Juliano was vandalised in Greece.

"Guess who? Mossad's behind this for sure," said one activist, as organisers attempted to ensure a round-the-clock guard on the remaining boats.

Footage of the mutilated propellers showed that the damage to both vessels was "remarkably similar," and had clearly been carried out "in a planned and professional way," they said.

Israeli-Swedish activist Dror Feiler, acting spokesman for the flotilla, said although there was no proof of Israeli involvement, the Jewish state was "the only party who would benefit" from such vandalism.

Israel has made no secret of its determination to prevent the flotilla reaching Gaza and has accused passengers of harbouring "terror activists" who are "looking for blood."

A similar attempt to break the blockade last May saw Israeli commandos kill nine Turkish activists and world leaders have urged the flotilla not to set sail because of the risk of further violence.

Israel has said it will stop any boats that attempt to enter Gaza "with minimal confrontation" -- prompting activists to accuse the state of attempting to stop the mission before it sets off.

"You don't think these attacks were done by amateurs do you?" said a passenger who asked not to be named. "We have to be very careful about what we say and do, we're under surveillance."

Activists? luck worsened when a French television company set to cover their departure discovered their hotel room had been broken into, their satellite equipment damaged and their computer tampered with.

"Of course it might just be an unconnected crime," the producer said. "But it seems a bizzare coincidence that someone should go to the effort of riddling our Macintosh computer with viruses."

Activists also believe there may have been an attempt to sabotage the petrol destined for the French boat, Louise Michel.

Greek police turned up at the Piraeus port near Athens on Thursday and stopped the crew just as they were about to refuel, banning them from doing so on the grounds that the boat's papers were not in order.

With the petrol in Greek custody, organiser Thomas Sommer said they were "getting a sample to send to our lawyer so that it can be tested" should something happen to the ship out at sea.

Then on Friday evening, campaigners feared Mossad agents were behind an attack on the captain of the boat as he returned to his Athens hotel at three in the morning.

Omeyya Seddik, a Tunisian passenger who was with the captain at the time, said around 10 people attacked them with motorcycle helmets, causing the seaman to pass out from the pain.

"Initially I thought it was just a racist attack not connected to the flotilla, but with everything else that's been happening to us, now I'm not so sure," he said, speaking through a split lip.

Activists have also reported suspicious individuals loitering near the French boat and apparently taking photographs of the vessel.

Their description appeared to match reports of men outside a central Athens hotel, filming flotilla passengers on their iphones.

Some campaigners said they believed Israel was attempting to wear them down psychologically to ensure the 10-boat flotilla would never set sail.

US boat organizers said a number of their passengers had their cellphones stolen, while another had her passport taken.

By Sunday it was clear that only four boats of the initial 10-strong flotilla would be able to take part in the mission, none of them legally because of a Greek ban on Gaza-bound vessels leaving the port.

The 300 or so activists from 22 countries who had signed up to take part still hope to find a way to cram onto the remaining vessels and defy the ban.

"That's unless another boat gets sabotaged first," Sommer said.