The boats are carrying 40 activists, 200 hearing aids and 5,000 balloons
Two boats carrying members of a US-based pro-Palestinian group have left Cyprus in an attempt to break Israel's blockade of the Gaza Strip.
The boats left the Cypriot port of Larnaca on Friday morning. The journey is expected to take about 30 hours.
The Free Gaza protest group said about 40 activists from 14 countries were on board the boats.
Israel imposed a blockade on Gaza in June 2007 when the militant group Hamas took control of the territory by force.
Since then, Israel has allowed in little more than basic humanitarian aid as a means of isolating Hamas and persuading militant groups to stop firing rockets into Israel.
The closure of Gaza's borders by the Israeli and Egyptian authorities has also meant that very few Gazans have been able to leave.
Before Free Gaza's boats set sail on Friday, the Israeli foreign ministry warned them to steer clear of the Gazan coastline, which it said was "the subject of an [Israeli Navy] advisory notice" that warns off foreign vessels from the "designated maritime zone".
"We assume that your intentions are good but, in fact, the result of your action is that you are supporting the regime of a terrorist organisation in Gaza," the ministry wrote in an open letter.
The two vessels - named Liberty and Free Gaza - are carrying 200 hearing aids for children and 5,000 balloons.
"No matter what happens we have already achieved our goal by proving that ordinary citizens with ordinary means can mobilise a defence of human rights for Palestinians," organiser Paul Larudee told the AFP news agency.
"We want people to see the Palestinian problem as one of human rights, not feeding them rice," he added.
The activists include Lauren Booth, sister-in-law of former British PM Tony Blair, who is now an international Middle East peace envoy. Also on board is left-wing Greek MP Tasos Kourakis.
Israel withdrew its settlers from Gaza in 2005, but it still controls its coast, airspace and borders, and, until a ceasefire with Hamas was agreed in June, carried out regular military operations in the territory.
However, correspondents say the truce has not improved the situation for Gaza's population, except to reduce the number of Israeli incursions and the number of rockets fired by Palestinian militants.