The 12 passengers are mostly from Sweden but also from Germany, Britain, Spain, France and Canada. They are in custody and will be put on flights home, an Israeli official said.
The SY Freedom for Gaza was carrying mainly medical supplies and two of its passengers were journalists, said an organizer from a group named Ship to Gaza which organized the trip, Reuters reported.
“Ship to Gaza demands that those who have been taken prisoner and the ship and its cargo are returned to the location where the ship was boarded and are allowed to continue their journey in international and Palestinian waters unmolested, in accordance with international law,” the group said in a statement.
“The IDF 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,” an Israeli military statement said.
Gaza has been under crippling Israeli blockade since 2007. More than two million Palestinians are packed into the Gaza Strip, which suffers deep economic hardship.
Hamas officials in Gaza had no immediate comment on the ship’s seizure. Similar attempts by activists to challenge the Israeli naval blockade of Gaza have been foiled before.
The most serious such incident was in 2010, when 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.
Also last Sunday, a Norwegian ship of activists trying to break the blockade on the Gaza Strip was stopped by Israel and forced to reroute toward Ashdod.
The captain of the Norwegian ship carrying 22 pro-Palestinian activists from 16 countries has accused Israel of violating the law by boarding the vessel in international waters and using violent force against its crew.
Norway also has asked Israel to explain the circumstances surrounding the arrests and "the allegations of excessive use of force," a Norwegian government spokesman told AFP on Thursday.
"We were arrested in international waters and we were closer to Egypt than Israel," the boat's captain, Herman Reksten, said early Thursday when he returned to Norway after being held for three days in an Israeli prison.
"Israel has broken all the rules, it's horrific that they board a Norwegian ship in international waters and force it to moor in Israel," he told Norwegian public broadcaster NRK.
The one-time fishing boat was carrying medication for people in Gaza.