The boat, named "Irene," left the port of Famagusta in the Turkish-held north of the divided eastern Mediterranean island in the early afternoon carrying eight activists, three of whom are crew members, and two journalists.
Reuven Moskovitz, an 82-year-old passenger who survived the Nazi Holocaust, told AFP he felt duty-bound to attempt the voyage, which is expected to take around 36 hours.
"It is a sacred duty for me, as a (Holocaust) survivor, to protest against the persecution, the oppression and the imprisonment of so many people in Gaza, including more than 800,000 children," Moskovitz said.
Yonatan Shapira, a former pilot for the Israel Defense Forces and crew member on the British-flagged sailing boat, said they were not looking for a confrontation.
"We have a policy of non-violence and non-confrontation," he said.
"But if the Israeli army stops the boat, we will not help them to take it to Ashdod," he said, referring to a port in southern Israel where other blockade runners have been taken after being stopped by the navy.
Israeli Defence Minister Ehud Barak has repeatedly warned that Israel will intercept any ship nearing Gaza, which is run by the Islamist movement Hamas.
In May, Israeli forces tried to stop a six-ship flotilla heading for Gaza but the raid went badly wrong, and nine Turkish activists were killed, prompting a wave of international condemnation.
"The boat's cargo includes symbolic aid in the form of children's toys and musical instruments, textbooks, fishing nets for Gaza's fishing communities and prosthetic limbs for orthopaedic medical care in Gaza's hospitals," said a statement from the organisers, Jews for Justice for Palestinians.
Richard Kuper, a member of the organising group, said that "the Jewish Boat to Gaza is a symbolic act of protest against the Israeli occupation of the Palestinian territories and the siege of Gaza, and a message of solidarity to Palestinians and Israelis who seek peace and justice.
"Israeli government policies are not supported by all Jews." Kuper said in the statement.
"We are banging our head on a very hard wall of hatred. Our hope is to make little cracks on that wall, so that in the end it will fall," said activist Rami Elhanan, who is also on the boat.
