Tens of thousands of people have joined conservative and far-right Spanish political leaders in Barcelona to protest against the Socialist party’s decision to consider an amnesty for those involved in the failed, unlawful and unilateral push for Catalan independence six years ago. Sunday’s demonstration, organised by the anti-independence group Societat Civil Catalana, was called after Catalan separatist parties said they would only consider supporting a new, socialist-led government following July’s inconclusive general election if they were offered an amnesty in return. Although the conservative People’s party (PP) finished first in the election, it fell well short of a parliamentary majority and failed to secure MPs’ backing to form a government during an investiture vote at the end of September. The Spanish Socialist Workers party (PSOE), led by the acting prime minister, Pedro Sánchez, now has King Felipe’s blessing to try to form a government but will be able to do so only with the votes of the hardline pro-independence Junts party, which is led by the former Catalan regional president Carles Puigdemont. Puigdemont, who fled Spain six years ago to avoid arrest over his role in masterminding the attempted secession, is one of hundreds of Catalan activists to be summoned before Spanish courts. Opposition politicians argue that Sánchez’s willingness to entertain an amnesty is further proof of his over-reliance on Catalan separatist parties and a cynical determination to remain in office. Addressing Sunday’s demonstration, the PP leader, Alberto Núñez Feijóo, described the proposed political deal as a “backwards step” that was opposed by most Spaniards. “We stand with democracy – and anything that amounts to a reactionary decision needs to be condemned by all democrats,” he said. “Catalonia isn’t a business or a society where the law isn’t respected, and the office of prime minister cannot be sold or traded.” Also present was Isabel Díaz Ayuso, the populist PP leader of the Madrid region who is often tipped as a possible successor to Feijóo. She said Sánchez was “trying, tyranically, to lead us down a blind alley” in return for a handful of votes. Santiago Abascal, the leader of the far-right Vox party – whose rise owes much to its fervent opposition to Catalan independence – called the amnesty “an assault on the constitution and on the hardworking, honourable people who pay fines and taxes and obey the law even when they don’t agree with it”. Organisers put attendance at the march at 300,000, while Barcelona police estimated that 50,000 people had taken part. A similar march in Madrid last month drew about 40,000 people. Salvador Illa, the leader of the Catalan branch of the Socialist party, said the protest had served only to raise tensions and worsen the political polarisation. “What we’re seeing once again today is the right hand-in-hand with the far right: Feijóo right there next to Abascal, stirring up fears,” he said. Catalonia’s pro-independence regional president, Pere Aragonès, described Sunday’s protest as a failure for the right and the far right and said it would only lead to a stronger push for independence. “Feijóo, Ayuso and Abascal have failed because the only message they have for Catalonia is one of vengeance, repression and hatred,” he said. Sánchez, meanwhile, has insisted that any step he takes will be in accordance with the constitution and “in the common interest”. He has also argued that his decision two years ago to grant pardons to nine Catalan independence leaders for their parts in the push for secession had helped calm the situation. The PSOE leader has until 27 November to put his candidacy before MPs in another investiture debate and vote. If he proves unsuccessful, parliament will be dissolved and Spain will return to the polls in mid-January for its sixth general election in nine years.