(CNN) -- Twenty-seven people from nine countries set sail Wednesday afternoon from Fethiye, Turkey, aboard two civilian boats en route to Gaza to challenge Israel's ongoing blockade of the territory.
"The message they carry is one of unity, defiance, and hope, in spite of Israel's policies that have physically separated Palestinians from each other," said the organizers of the event, called Freedom Waves to Gaza.
The organizers said they did not publicize their effort prior to setting sail because of "Israel's efforts to block and sabotage Freedom Flotilla II last July."
The boats -- according to organizers, carrying $30,000 in medicines and passengers committed to "nonviolent defense of the flotilla and Palestinian human rights" -- are expected to arrive Friday afternoon in Gaza, organizers said in a statement.
"Israel's siege of Gaza is untenable and it's a moral responsibility to put an end to this injustice," said Majd Kayyal, a Palestinian philosophy student from Haifa on board the Tahrir, one of the boats, in the statement.
"We are here to end the siege on Gaza and to tell the world to end the Israeli occupation of Palestine," said Kit Kittredge, a U.S. peace activist aboard the Tahrir, a Canadian vessel. "Our American government is complicit in this siege on Gaza, we say we don't want our tax dollars to be apartheid tax dollars and support the siege on Gaza."
He said he and the others aboard the ship are non-violent, have no weapons and will not fight.
A statement the group said had been signed by Palestinian youths urged the international community and the United Nations in particular "to take urgent action to protect this mission as well as to end its compliance with Israel's criminal blockade of Gaza."
This week, Palestinian activists in the West Bank and inside Israel will organize solidarity actions, including a presence outside the U.N. compound in Ramallah and rallies in the West Bank.
The Israel Defense Forces said the Israeli Navy "has completed the necessary preparations in order to prevent them from reaching the Gaza Strip.
"The purpose of this attempt is to create a provocation against the State of Israel, to break the maritime security blockade on Gaza, and to undermine Israel's security."
It described the blockade as "legal and in accordance with international law" and said that humanitarian aid can be sent to Gaza "via the existing land crossings and in coordination with the Israeli authorities."
Last year, nine civilians died when Israeli commandos intercepted an aid vessel bound for Gaza.
According to the activist organizers, Wednesday's mission is the 11th attempt to reach Gaza via the sea, with five missions arriving safely in Gaza between August and December 2008, and the others intercepted by Israel.
CNN's Kevin Flower contributed to this story