The boats in the so-called Global Sumud Flotilla want to "break the illegal siege" of the Gaza Strip by opening a humanitarian corridor by sea.
It should not be up to us, says Thunberg before departure according to several media.
It is the responsibility of countries, our governments and elected representatives, to act to uphold international law, prevent war crimes and genocide. It is their legal duty, and they are failing.
This is Thunberg's second attempt to reach the Gaza Strip. In June, she and other activists were stopped by Israel in international waters west of Gaza, taken into Israeli custody and subsequently sent home.
The Global Sumud Flotilla describes itself as an independent group with no ties to any specific country or political party. Several more ships will join the voyage as it moves east through the Mediterranean. According to their own statement, they will constitute "the largest solidarity action" in history.
On board are humanitarian aid and activists, volunteers and politicians from over 40 different countries, writes EFE. Alone from Genoa, over 300 tons of emergency aid are being sent, according to the news agency.
In addition to Swedish Greta Thunberg and Lorena Delgado Varas, who was recently expelled from the Left Party after allegations of anti-Semitism, the flotilla includes, among others, Barcelona's former mayor Ada Colau and the Portuguese MP Mariana Mortágua.
Arrival in the Gaza Strip is expected in mid-September.
