The enormous rally through the city saw marchers call for the wealthy northeast region to drop its bid for independence, with participants declaring themselves loyal to both Spain and Catalonia.
Organisers claimed 950,000 people took part, although Spanish police put the number at 350,000.
Spain is on the brink of a major political crisis, with the Catalonia president Carles Puigdemont set to make a statement to the regional parliament on Tuesday about the region’s future.
While he was initially expected to declare unilateral independence, MP Marta Pascal said it would instead be a “symbolic’’ statement.
Ms Pascal told the BBC that Mr Puigdemont would recognise the results of the referendum, before embarking on a discussion about the path to independence.
In response, Spanish Prime Minister Mariano Rajoy has made clear he is considering revoking Catalonia’s autonomous status.
Protesters hold Spanish flags during a demonstration called by “Societat Civil Catalana” (Catalan Civil Society) to support the unity of Spain in Barcelona. Picture: AFP / PAU BARRENASource:AFP
Mr Rajoy said he was not rule out anything “within the law’’ to stop Catalonia breaking away from Spain, which has not been gripped by such serious political turmoil since the death of the dictator Francisco Franco in 1975.
“The government will ensure that any declaration of independence will lead to nothing,” Mr Rajoy told Spanish newspaper El Pais.
The high stakes political manoeuvring comes a week after a referendum saw 2.3 million Catalonians vote to secede from Spain — with 90 per cent supporting independence. The turnout was 43 per cent.
Spanish police had tried to shut down the vote after the High Court ruled it illegal, and almost 900 people were injured in the violent clashes with police than ensued.
The Spanish courts also shut down a Catalonian parliamentary sessions planned for today.
Spanish national police and the military have been deployed to the region and are likely to remain in place for the foreseeable future.
Rallies for and against the independence push have been held across Spain over the weekend.
In Madrid, anti-independence protesters who rallied at a march organised by a far-right party were seen giving fascist salutes as they called for Spain to remain united.