Swedish climate activist Greta Thunberg and Irish actor Liam Cunningham are among the crew of the Global Sumud Flotilla, which is bringing food, water and medicine.
Delegations from 44 countries are taking part, and more vessels from Italy, Tunisia, Greece and Sicily are expected to join before the flotilla reaches Gaza around 14 or 15 September. Past efforts to breach the blockade were stopped by Israel.
The mission comes as Gaza faces severe famine, with 332 Palestinians, including 124 children, reported dead from malnutrition during the 23-month war that has killed more than 63,000 people.
