The Israeli navy on Tuesday boarded and seized a ship carrying pro-Palestinian activists that was heading to the Gaza Strip in defiance of Israel's blockade, a military spokesman said.
"In the last hour an Israeli navy force intercepted, boarded and took control of the cargo boat 'Arion' ... as it was illegally attempting to enter the Gaza Strip," the spokesman said late in the afternoon.
The military said the ship, which was sailing under a Greek flag, was the same vessel known as the "Spirit of Humanity",which was sailing to Gaza to protest the Israeli blockade on the Hamas-ruled territory.
The spokesman said the navy had been contacting the boat since Monday evening and warned that it would not be allowed to enter Gazan waters "because of security risks and an existing naval blockade".
After the navy boarded the converted ferry it towed the vessel toward the nearby Israeli port of Ashdod, the spokesman said, adding that the activists on board would be handed over to authorities.
He said no shots were fired during the incident.
The boat was sent by the U.S.-based Free Gaza Movement, which said it was carrying a "symbolic" cargo of humanitarian aid to pressure governments and international relief groups to do more to rebuild Gaza.
Greece quickly protested the seizing of the Greek-flagged vessel, saying "we remonstrated with the Israeli side and asked for the ship, the crew and the passengers to be released immediately", foreign ministry spokesman Grigoris Delavekouras said in a statement.
"As we have said in the past, all legal activity by non-governmental organisations must enjoy freedom," he said.
Activists have made five successful missions to Gaza from Cyprus since August 2008 - but boats have been turned back twice by Israeli forces.
Israel has maintained a strict blockade on the Palestinian territory since the Islamist movement Hamas seized power in 2007, and it launched a massive 22-day offensive on Gaza in December in a bid to halt rocket fire.
On Dec. 30, shortly after the Gaza war erupted, an Israeli navy vessel collided with a Free Gaza boat, almost sinking it, in what activists called a "deliberate ramming".
Earlier on Tuesday, the Free Gaza movement said that Israeli warships had surrounded the Spirit of Humanity and threatened to open fire if it did not turn around.
At one point, the Israelis jammed the boat's instruments, blocking their GPS, radar and navigation systems "in direct violation of international maritime law," the group said.
The Israeli foreign ministry said the boat's owners had lied before departure from Cyprus about the vessel's destination, saying it was bound for Port Said in Egypt.
The ministry also said that under the 1993 autonomy accords struck with the Palestinians, Gaza's territorial waters, like its land borders, were Israel's responsibility.
Former U.S. congresswomen Cynthia McKinney, one of 21 activists aboard the Spirit of Humanity, expressed outrage.
"We demand that the Israeli government call off their attack dogs," the Free Gaza Movement quoted her as saying a few hours before the vessel was intercepted.
Irish Nobel peace prize laureate Mairead Maguire, for her part, said U.S. President Barack Obama "has called upon the Palestinians to abandon violence but Israel is denying them the right to non-violently resist the siege of Gaza".