A flotilla bound for Gaza carrying aid and pro-Palestinian activists set sail Monday from Tunisia after repeated delays, aiming to break Israel's blockade and establish a humanitarian corridor to the Palestinian territory.

"We are also trying to send a message to the people of Gaza that the world has not forgotten about you," Swedish climate activist Greta Thunberg said before boarding in the northern port of Bizerte.

"When our governments are failing to step up then we have no choice but to take matters into our own hands," she told AFP.

Around 20 boats that had sailed from Barcelona converged in Bizerte, with the last vessels leaving at dawn, an AFP journalist reported.

Yasemin Acar, who helps coordinate the flotilla from the Maghreb, posted images on Instagram of boats also departing in the early hours.

"The blockade of Gaza must end" and "We are leaving for solidarity, dignity and justice", the caption said.

The vessels had transferred to Bizerte after a turbulent stay in Sidi Bou Said near Tunis.

The Global Sumud Flotilla said two of its boats were targeted by drone attacks on consecutive nights last week.

After the second incident, Tunisian authorities denounced what they called a "premeditated aggression" and announced an investigation.

European Parliament member Rima Hassan, who like Thunberg was detained aboard the Madleen sailboat during a attempt to reach Gaza in June, said she feared further attacks.

"We are preparing for different scenarios," she said, noting the most prominent figures had been split between the two largest coordinating boats "to balance things out and avoid concentrating all the visible personalities on a single vessel".

The departure had been repeatedly postponed due to security concerns, delays in preparing some of the boats and weather conditions.

The flotilla, which also includes vessels that left in recent days from Corsica, Sicily and Greece, had originally planned to reach Gaza by mid-September, after two earlier attempts were blocked by Israel in June and July.