A flotilla of ships sailing towards Gaza with aid and activists on board has left Cyprus and will reach its destination on Monday, organisers say.
But Israel says it will stop the boats, calling the campaign a "provocation intended to delegitimise Israel".
The Palestinian territory has been under an Israeli and Egyptian economic blockade for almost three years, with only limited humanitarian aid allowed.
The activists, from the Free Gaza Movement, want to break the blockade.
Israel imposed the measures after the Islamist movement Hamas took power in Gaza.
Hamas has fired thousands of rockets into Israel over the past decade.
For days, human rights activists aboard the flotilla of ships have been saying they are due in Gaza soon, but they are now running days late.
Organisers confirmed that they had left Cyprus on Sunday afternoon, after confusion over their exact plans.
Greta Berlin, of the Free Gaza Movement, told the BBC that the campaign was "extremely well organised".
She told the BBC: "A lot of that confusion is done on purpose because why should we telegraph to the Israeli navy... exactly when it is that we are going to come?"
She said the activists would try to negotiate their entry into Gaza's waters, amid reports that the Israeli navy intends to tow the ships to the nearby city of Ashdod and deport all of those on board.
Israeli government spokesman Mark Regev said the activists were "masquerading as human rights activists" while trying to make a political point.
He said both Israel and Egypt had offered to take the aid into Gaza, but that the activists had shown no interest.
"It appears they're putting their radical politics above the well-being of the people of Gaza," he said.
The BBC's Jon Donnison in Gaza City says there is much political spin being put on the story by both sides.
Hamas Prime Minister Ismail Haniya says any Israeli action to stop the ships will be an act of "piracy".
Israeli government press officers have been briefing journalists that the aid flotilla is not necessary. Israel says it allows 15,000 tons of aid into Gaza every week.
But the United Nations, which calls the blockade a "Medieval siege", says this is only a fraction of what is needed and less than a quarter of what was coming into Gaza before the blockade was enforced.