"It is not a vaccine but an experimental treatment and in fact there is a lot of coercion," said protester Ruben, who is refusing to receive the vaccine to get the digital pass.
Protesters marched in the Catalan capital chanting: "It is not a vaccine. It is an experiment" and holding signs against coronavirus certificates and vaccines.
Catalonia postponed the introduction of mandatory COVID passes last Friday (November 26) after thousands of people attempted to download the certificate from a local government website and crashed the system.
With a nationwide vaccination rate of nearly 80%, Spain had been largely spared the latest wave of contagion sweeping Europe that has pushed countries like Germany and Austria to reintroduce tough restrictions on travel and socialising.
Spain's 14-day coronavirus infection rate rose again on Thursday (December 2) to 234 per 100,000 people, having doubled since Nov. 19 - but it remains substantially lower than in some EU countries such as the Netherlands, Belgium or Germany.