"They forcibly boarded the ship, detained all our passengers and illegally took them to Israel against their will. This is a kidnapping. This is the act of piracy at sea," Ramzi Kyzia, an activist from the Free Gaza Movement, told a news conference in Cyprus.
"The Israeli navy made the choice to come out and intercept us and forcibly board us and kidnap 21 international human rights workers and journalists."
The activists, who were carrying humanitarian supplies, had set off from Cyprus in an attempt to break the Israeli blockade of the Gaza Strip, which prevents many basic supplies reaching the 1.5 million Palestinians in the territory.
Among those detained with Maguire were Cynthia McKinney, a US congresswoman, and two Al Jazeera journalists.
Yigal Palmor, the Israeli foreign minister, had said that those who signed an undertaking to return home voluntarily could be released immediately and repatriated on the first available flight.Deportation orders
But Maguire told Al Jazeera that some of the activists were considering applying to extend their detention in order to mount a legal challenge against the actions of the Israeli authorities.
"We have been issued with deportation orders which have expired today," she said.
"We refuse to be criminalised because we wanted to go to the aid of the people of Gaza."
Israel imposed the crippling blockade of the Gaza Strip in 2007 after the Hamas movement seized full control of the territory after pushing out Fatah security forces loyal to Mahmoud Abbas, the Palestinian president.
International calls for the siege to be eased have mounted since Israel fought a 22-day offensive in the territory in December and January, increasing the hardships faced by the Gazans.
Israel said that the war was aimed at ending rocket attacks by Palestinian fighters.