700 activists from around world vow to deliver 10,000 tonnes of humanitarian aid to break the blockade of Gaza.
Israel has cautioned that the Freedom Flotilla would be stopped, if necessary by force.
The nine-ship flotilla is by far the largest fleet of aid to try to reach the coastal Palestinian territory since Israel imposed its siege on it in 2007.
He restated UN opposition to the siege of Gaza and the lack of material to meet "basic needs, begin reconstruction, and revive economic life".
After the Israeli army announced a detention centre at Ashdod port for holding the activists, Greta Berlin, one of the flotilla organisers, said: "We have the right to sail from international waters into the waters of Gaza.
"The only illegal presence in the area is Israel."
Berlin said the Freedom Flotilla was on schedule to arrive in the Gaza Strip on Saturday with more than 10,000 tonnes of supplies, including water-filtration units and pre-fabricated homes.
Catherine Ashton, the EU foreign policy chief, issued a statement on Friday, calling for an immediate end to Israel's blockade on Gaza.
"We would like to reiterate the EU's call for an immediate, sustained and unconditional opening of crossings for the flow of humanitarian aid, commercial goods and persons to and from Gaza," she said.
"The continued policy of closure is unacceptable and politically counterproductive.
"The EU remains gravely concerned by the humanitarian situation in Gaza."
Israel has vowed to divert the ships to the southern Israeli port of Ashdod.
It has said that Israelis on board would be arrested, Palestinians would be questioned by the Israeli secret service, and foreign nationals deported.
Hanin Zuabi, a member of the Israeli parliament who is on board the flotilla, told Al Jazeera that the activists intend to reach Gaza regardless of plans to stop them.
"If the Israelis try to stop us, this will be a huge diplomatic and political crises for them," Zuabi said.
"We have 50 states participating in this and are sending a very clear message to Israel - the international community is not accepting the siege on Gaza."
Peace laureates aboard
Al Jazeera's Jamal Elshayyal, on board the flotilla, said the activists travelling in the convoy included European parliamentarians, former US diplomats and Nobel peace laureates.
Berlin, the flotilla organiser, said: "This mission is not about delivering humanitarian supplies, it's about breaking Israel's siege on 1.5 million Palestinians."
Fintan Lane, an Irish activist, said that they were determined to break Israel's blockade and will not be intimidated.
"The people of Gaza have a right to access to the outside world and the right to determine their own future," Lane said.
Huwaida Arraf, one of the organisers from the Free Gaza Movement, said: "Israel should not be under any illusion whatsoever that their threats or intimidation will stop us or even that their violence against us will stop us."