Tunisian fishermen have prevented a ship carrying far-right activists from docking, dealing a blow to their mission to disrupt the flow of refugee and migrant boats from Africa to Europe.
C-Star, chartered by French-based group Génération Identitaire (GI), was unable to berth in Zarzis.
GI says NGOs active in the Mediterranean collude with people traffickers, and has pledged to target boats run by the organisations to try to rescue refugees and migrants.
But the Zarzis fishermen said the anti-immigration activists were racists.
They vowed not to let the C-Star refuel if it landed. The vessel was expected to try another Tunisian port today.
"It is the least we can do given what is happening out in the Mediterranean," Chamseddine Bourassine, head of the local fishermen's organisation, told AFP.
"Muslims and Africans are dying."
A port official who asked to remain anonymous said: "Us let in racists here? Never."
Meanwhile, humanitarian groups say any attempt to turn refugee and migrant boats back to Libya could be very dangerous and illegal under international law.
About 600,000 people have been rescued from traffickers' boats and taken to Italy since the beginning of 2014.
More than 10,000 people have died attempting to cross the Mediterranean over the same period.
Earlier this month rights group Amnesty International accused the EU of mostly leaving it up to sea rescue charities to save refugees and migrants.
At the same time, NGOs have come under criticism from the Italian authorities, who have threatened to stop vessels of other countries from bringing migrants to Italian ports.
Italy's parliament has approved a plan to send naval boats to Libya as part of its efforts to stop migrants crossing the Mediterranean, and is asking NGO rescue ships to abide by a code of conduct.
Génération Identitaire - Generation Identity, in English - is a right-wing group based in France and Austria] and known for its relatively young and tech-savvy members.
It is fiercely anti-immigration and anti-Muslim, and aims to stop mass migration to Europe.