A flotilla of 10 vessels set off for Gaza from Sicily, southern Italy, on Saturday, with 60 people on board including elected officials from nine nations, said organisers.
A statement from the Freedom Flotilla Coalition (FFC) and the Thousand Madleens to Gaza (TMTG) said they intended to "break the illegal Israeli blockade" to deliver aid to the Palestinian territory.
"For the most part, our boats carry medical supplies, dry food and school equipment, as this was highlighted as some of the biggest priorities by Palestinians on the ground," they added.
They intend to join the Global Sumud Flotilla which is also bound for Gaza carrying aid supplies.
On board are elected officials from the European Parliament and from countries including Belgium, France, Ireland, Spain and the United States.
On Wednesday, the United Nations called for an investigation into alleged drone attacks off Greece against the Global Sumud Flotilla, which activists have blamed on Israel.
Agence France-Presse