Communist Juan Manuel Sanchez Gordillo made headlines two years ago after organising Robin Hood-style raids on supermarkets and handouts of the stolen food to poor families.
Now the left-wing union he leads is preparing for an invasion of the rock on 29 August.
Mr Gordillo, mayor of the town of Marinaleda near Seville as well as head of the Andalucian Workers Union, has claimed they will cross the border peacefully. Around 2,000 people are expected to take part although the union has 20,000 members and is hoping for support from Spaniards who work on the rock or are visiting.
A union spokesman described it as a protest against tax havens and British sovereignty of Gibraltar and a defence of Spanish fishermen who they claim have been badly hit by an artificial reef built off the rock.
Mr Sanchez, who boasts of running his town like a communist utopia, said: “It will be a peaceful demonstration but one meant to get the ball rolling on winning it back.”
Police in Gibraltar are expected to increase security at the border on 29 August.