A flotilla carrying humanitarian aid and activists, including Swedish climate campaigner Greta Thunberg, is due to leave from Barcelona on Sunday to try to "break the illegal siege of Gaza", organisers said.

The vessels will set off from the Spanish port city to "open a humanitarian corridor and end the ongoing genocide of the Palestinian people", said the Global Sumud Flotilla.

The flotilla is expected to arrive at the war-ravaged coastal enclave in mid-September.

"This will be the largest solidarity mission in history, with more people and more boats than all previous attempts combined," Brazilian activist Thiago Avila told journalists in Barcelona last week.

Organisers say that dozens of other vessels are expected to leave Tunisian and other Mediterranean ports on September 4.

Activists will also stage simultaneous demonstrations and other protests in 44 countries "in solidarity with the Palestinian people", Thunberg wrote on Instagram.

As well as Thunberg, the flotilla will include activists from several countries, European lawmakers and public figures such as former Barcelona Mayor Ada Colau.

The Global Sumud Flotilla describes itself as an independent group not linked to any government or political party. Sumud means "perseverance" in Arabic. (AFP)