Some 15,000 people according to police -- between 75,000 and 200,000 according to organisers -- took part in the protest on behalf of "Tabernia" which encompasses the parts of Catalonia most opposed to splitting from Madrid.
The streets were filled with Spanish flags as well as those of the fictive region as the protesters marched through central Barcelona and to the Catalan parliament, where secessionists are in the majority.
"We are here because we are fed up, fed up of the independantist process and that a minority is leading us towards the brink," said Mari Carmen Guerrero, a 33-year-old protester with a Tabarnia flag round her neck.
Among those joining Sunday's march was Alberto Fernandez Diaz, regional head of Spain's ruling Popular Party and Javier Ortega Smith, head of the ultra-conservative Vox party
Last October Catalonians voted, in a referendum declared illegal by Madrid, for independence from Spain.
Weeks later, separatist lawmakers declared independence on October 27th.
The Spanish government moved in immediately, stripping Catalonia of its prized autonomy, sacking its separatist government, dissolving its parliament and calling snap elections, which the separatists won.
The name Tabarnia was dreamed up in recent years to turn the separatists' arguments back at them.
The word comes from the names of the coastal provinces of Tarragona and Barcelona.
In January, Catalan author and satirist Albert Boadella, a harsh critic of the region's ruling nationalist CiU party, was chosen as "Tabarnia's president-in-exile', a poke at Catalonia's deposed leader Carles Puigdemont who is wanted for sedition in Spain, and said he was willing to govern Catalonia from Belgium, where he is in self-imposed exile.
People holds masks depecting actor Albert Boadella, president of the Tabarnia movement,. Photo: AFP
"It's a joke to make the separatists realise that their argument are ridiculous and the entire process absurd," said Jose Luis Cortes, a 72-year-old retiree who was among the marchers.
"But if they go ahead, maybe it will stop being a joke," he added, "because if they want to leave Spain and Europe, do it, but we will stay".
The supporters of Tabarnia have not only devised their own flag but also have their own anthem and even currency and say they are prepared, if necessary, to begin a legal fight for recognition as a region separate from Catalonia.
Want to understand what Tabernia is all about? Watch this explanatory video:
Pedimos A TODOS LOS ESPAÑOLES que en los próximos días se pongan de avatar en sus perfiles la bandera de #Tabarnia para demostrar a los indepes que HA EMPEZADO UNA NUEVA ETAPA y que toda España está con nosotros. En whatsapp, facebook, twitter, instagram, telegram... ¡vamos! 😀 pic.twitter.com/KbumMjYQ0U— Tabarnia Oficial (@Bcnisnotcat_) January 14, 2018