A Swedish ship due to join an upcoming Gaza-bound aid flotilla has been sabotaged in the Greek port of Piraeus, organisers say.
In a statement, they said "hostile divers had destroyed the propeller house and cut the propeller shaft" of the vessel Juliano on Monday.
The ship is part of the 10-vessel Freedom Flotilla II that is expected to set sail from Greece and elsewhere for the Gaza Strip in the coming days in a bid to break Israel's blockade of the Palestinian territory.
About 350 pro-Palestinian activists from 22 countries are likely to participate.
Israel insists the latest flotilla is a "dangerous provocation" and has vowed to intercept it.
Organisers of the flotilla, however, remain defiant and said the Juliano would be ready to sail within one or two days after being repaired. They said they had documented the sabotage with their own camera-equipped divers.
"We are sad that people are doing such things but we are determined to continue to Gaza," Dror Feiler, one of the organisers, told Al Jazeera from aboard the Juliano.
"We will not be frightened by Israel, and we are going to continue. Our friends from all around the world are with us, and we are all going to Gaza."
Mattias Gardell, a spokesperson for Ship to Gaza Sweden, also condemned the act of sabotage.
"It's one thing for a foreign power to press the Greek government to delay our voyage with red tape. It is quite another thing for enemy agents to operate on Greek territory.
"It is high time for the international community to put their foot down and say: Enough!"
Israeli newspaper Haaretz quoted senior Israeli officials as saying that "radical elements" among the flotilla activists had stated an intention to "spill the blood of Israeli soldiers".
According to Tel Aviv daily Yedioth Aharonoth, military sources said participants of the flotilla were planning to pour chemicals, such as sulfur, on Israeli soldiers.
But according to activist Feiler, the goal of the flotilla is to "come in peace" to Gaza.
"We are shocked by the Israeli actions, and their propaganda that we have weapons and acid and are going to attack Israeli soldiers, when we are all dedicated to peace," Feiler said. "We will not throw objects or attack them in any way."
The flotilla comes a year after another aid shipment was intercepted by Israeli commandos. At least nine activists were killed when commandos stormed Mavi Marmara, the Turkish-owned lead aid ship.
Besides the Juliano - named after Juliano Mer-Khamis, the Arab-Israeli actor, director and political activist who was shot dead in the West Bank town of Jenin in April - boats from Greece, France, Italy and Spain are also among those joining Freedom Flotilla II. Two cargo vessels will carry medicines, a fully equipped ambulance car, and cement.
A number of journalists are among those taking part in the bid to break Israel's five-year naval blockade on the Gaza Strip, which is home to 1.5 million Palestinians.
Ban Ki-moon, the United Nations secretary-general, and several international leaders have urged the flotilla not to set sail, and the US has warned its nationals not to join the attempt to break the embargo.
The border has remained largely shut since June 2006, when Israel imposed a tight blockade on the coastal territory after Palestinian fighters snatched Israeli soldier Gilad Shalit, who is still being held.
The UN has called the blockade illegal and repeatedly demanded it be lifted. A ban on civilian goods and foodstuffs was eased last year but many restrictions remain in place.