But Israel insisted it would prevent the three cargo ships and five passenger boats from reaching the Islamist-run Gaza Strip, calling the blockade-busting bid a "cheap political stunt."
The foreign ministry said it summoned the ambassadors of Turkey, Greece, Cyprus, Sweden and Ireland -- the countries from which the ships set sail -- and warned them Israel "issued warrants that prohibit the entrance of the vessels to Gaza."
The flotilla "is about to break international law," the ministry's Director General Yossi Gal told the diplomats, adding the coastal enclave "is under the control of a terrorist organisation, which does not prioritise the well-being of the citizens of Gaza."
He also said Israel "declares the flotilla an absolute provocation" and that there is no shortage of humanitarian aid in Gaza.
Israel has vowed to divert the ships to the southern Israeli port of Ashdod, where it will detain the activists before deporting them.
Part of the port has been cordoned off and prepared to deal with the activists. Chemical toilets have been set up as well as large, air-conditioned tents housing immigration booths and areas for people to be searched.
Containers blocked off what appeared to be a holding area, which was covered by tarpaulins to provide shade.
Gal suggested the organisers should voluntarily head to Ashdod to unload the supplies so Israel or humanitarian agencies can deliver them to Gaza overland.
Flotilla organisers rejected the offer.
"We are taking 10,000 tonnes of material that Israel refuses to allow into Gaza," Berlin said, adding the cargo includes water filtration units, pre-fabricated homes and crayons for children.
"This mission is not about delivering humanitarian supplies, it's about breaking Israel's siege on 1.5 million Palestinians," she said.
Irishman Fintan Lane echoed the sentiments.
"We are determined to break Israel's blockade and will not be intimidated," said Lane, one of 750 activists from around the world, including dozens of public officials from European and Arab countries, that organisers say are heading towards Gaza.
"The people of Gaza have a right to access to the outside world and the right to determine their own future," said Lane.
Israel and Egypt have sealed Gaza off to all but basic humanitarian aid since Hamas -- which is blacklisted as a terrorist group by the European Union and the United States -- seized power in June 2007.
Hamas on Thursday said Israel's threats to intercept the flotilla amounted to "Zionist piracy."
"The occupation's threat to prevent the Freedom Flotilla from arriving in the besieged Gaza Strip is Zionist piracy and a violation of international law," senior Hamas leader Ismail Radwan said in a statement.
"The occupation is concerned about these ships... because they grant legitimacy to engagement with the Palestinian government and confirm that the attempts to isolate Hamas have failed," he added.