"Ten Israeli warships forced the boat to head for (the Israeli port) of Ashdod by force, but without raiding the ship," Amjad al-Shawa, a Gaza-based organiser, told AFP.
"They surrendered because they were surrounded. They had no choice," he added.
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Earlier, an Israeli warship ordered the boatload of Jewish activists to change course as they tried to enter territorial waters around the Gaza Strip, one of the passengers told AFP.
"They contacted us and asked us who we are, where we come from and our destination," Yonatan Shapira told AFP by satellite telephone.
"They said we were approaching an area under naval blockade and told us to change course," he said.
The sound of a voice over a megaphone urging the vessel, "Irene", to "change course" could be heard in the background.
Shapira said the British-flagged sailing boat, which is carrying seven Jewish activists from Europe and the United States and two journalists, was some 20 miles from the Gaza coast and had been shadowed by the navy ship.
Israel has vowed to halt the vessel and divert it to the southern Israeli port of Ashdod if it enters Gaza waters.
Shapira had earlier said the boat hoped to reach Gaza within a few hours. The activists have insisted they are not looking for a confrontation with Israeli forces.
"We have a policy of non-violence and non-confrontation," Shapira, a former Israeli pilot, told AFP on Sunday. "But if the Israeli army stops the boat, we will not help them to take it to Ashdod."
In the past, Israel has said it would deliver any humanitarian cargo to Gaza overland after towing such boats to Ashdod.
In May, Israeli forces intercepted a six-ship flotilla heading for Gaza but the raid went badly wrong and nine Turkish activists -- including one with US citizenship -- were killed, prompting a wave of international condemnation.
Israel said its troops resorted to force only after they were attacked while rappelling onto the deck of the lead ship. Pro-Palestinian activists on board say the soldiers opened fire as soon as they landed.
The voyage of the Irene is organised by the London-based Jews for Justice for Palestinians.
Prominent British supporters listed on its website include humourist and actor Stephen Fry and Marion Kozak, the mother of newly-elected Labour Party leader Ed Miliband and of former foreign minister David Miliband.
On board the Irene are 82-year-old Holocaust survivor Reuven Moskovitz and Rami Elhanan, an Israeli whose daughter Smadar was killed in a 1997 Palestinian suicide bombing in Jerusalem.
With them are a German nurse, British and US peace activists, Shapira's brother and a reporter for Israel's Channel 10 television.
Israel and Egypt sealed Gaza's borders after Palestinian militants captured an Israeli soldier in June 2006 and tightened the blockade a year later when the Islamist Hamas movement seized power, allowing in only humanitarian aid.
Israel eased the closures to allow in all purely civilian goods in the aftermath of the deadly flotilla raid, but still restricts dual-use items such as construction materials that could be used to build militant fortifications.
