BARCELONA — A flotilla of boats headed for Gaza carrying 350 pro-Palestinian activists — including climate change campaigner Greta Thunberg — has departed from Barcelona.

Around 20 vessels displaying Palestinian flags left the Spanish port at 19:00 local time on Monday, stocked with medical and food supplies.

The flotilla first attempted to set sail on Sunday, but had to return to port due to stormy weather.

The aim of the mission is to "break Israel's illegal siege on Gaza", the organisers said — though a previous attempt to reach Gaza by sea was intercepted by Israeli forces.

The Global Sumud Flotilla Mission plans to launch in two waves — the first from Barcelona and the second from Tunis on 4 September.

Activists hope the boats will converge in the Mediterranean before sailing towards Gaza, a trip that is estimated to take seven or eight days.

"We are sailing again to break the siege and open up a humanitarian corridor," Thunberg said in a video message.

Punk singer Bob Vylan — who provoked controversy for leading a chant of "death to the IDF [Israel Defence Forces]" at Glastonbury — spoke at the flotilla's departure on Sunday.

He called the activists as "brave individuals" who were "attempting to do what should have been done by government intervention a long time ago".

Israeli authorities have characterised a previous attempt to sail aid to Gaza as a publicity stunt that offered no real humanitarian assistance.

In March, Israel introduced a nearly three-month total blockade on goods entering Gaza, before allowing a limited amount of goods back into the territory in late-May following international pressure. a UN-backed body has since declared there is a famine in parts of Gaza.

Israel has since tried to impose its own distribution system through the controversial Gaza Humanitarian Foundation, which has been criticised by aid agencies.

In June, a yacht carrying Thunberg and several other activists — and a symbolic amount of humanitarian aid — was intercepted by Israeli forces as it tried to reach Gaza.

Israeli authorities escorted the activists to the port of Ashdod before deporting them from the country.

In 2010, Israeli commandos killed 10 people when they boarded Turkish ship Mavi Marmara which was leading an aid flotilla towards Gaza. — BBC