The 12 passengers are mostly from Sweden but also from Germany, Britain, Spain, France and Canada (Photo courtesy of Ship to Gaza)
The Israeli navy has seized a Swedish-flagged activist boat named Freedom for Gaza which was attempting to breach its more than decade-long blockade of the besieged enclave, the second in less than a week.
Ship to Gaza, the organisers of the flotilla, said the boat was carrying medical supplies and was intercepted in international waters late on Friday.
"The demands of Ship to Gaza are that the ship with its crew and cargo will be returned to the site of the boarding, and that they will be allowed to go in peace through international and Palestinian waters in accordance to international law," they said in a statement.
"This is a demand that the eleven years-long illegal and destructive blockade on Gaza will be lifted at last."
In a statement, the Israeli military said the vessel was carrying 12 people and had been taken to the port of Ashdod.
"The ship was monitored and was intercepted in accordance with international law," the statement said.
"The [military] clarified to the ship's passengers that they are violating the legal naval blockade and that any humanitarian merchandise can be transferred to Gaza through the Port of Ashdod."
The people on board were taken for "further inquiry", it said.
The 12 passengers are mostly from Sweden but also from Germany, Britain, Spain, France and Canada.
A spokeswoman for the Israeli Immigration Authority said the passengers are in custody and will be put on flights home.
Freedom was the second boat of the "Freedom Flotilla" to be intercepted en route to break the blockade on Gaza, organisers said.
Four boats left from Scandinavia in mid-May and stopped in some 28 ports along the way.
One of the other boats was damaged during a storm and had to return to port in Palermo, Sicily, while the fourth boat never left Sicily due to technical issues.
'Hit on head'
On Sunday, the Israeli navy intercepted a Norwegian-flagged activist boat that was part of the flotilla.
Once on board the Al Awda (The Return), authorities arrested the 22 passengers and crew. The boat was then towed into Ashdod.
Torstein Dahle, head of the group Ship to Gaza Norway, told the Reuters news agency that the captain of the Al Awda boat had been struck on the head by Israeli soldiers who ordered him to sail for Israel, but no one was seriously injured.
Since 2008, activists from all over the world have sailed 31 boats in attempts to challenge the Israeli naval blockade on Gaza.
UN officials have called for the blockade to be lifted, citing deteriorating humanitarian conditions in the Palestinian enclave run by Hamas where 80 percent of the two million population are dependent on aid.
In 2010, Israeli commandos raided a Turkish ship, the Mavi Marmara, that was leading a flotilla to Gaza.
Ten activists aboard were killed, leading to a major diplomatic crisis between Israel and Turkey.