Spain’s congress has approved the controversial and divisive Catalan amnesty bill that regional separatists demanded in return for helping the country’s Socialist-led coalition government back into office after last year’s inconclusive general election.

The passing of the bill, which was approved by 178 votes to 172 in Spain’s 350-seat parliament, will come as a relief for the prime minister, Pedro Sánchez, who has gambled his political future on the concession.

Thursday’s vote was held amid heightened political tensions, and a day after Catalonia’s president, Pere Aragonès, called an early regional election after the defeat of his government’s budget in the Catalan assembly.

Although the conservative People’s party (PP) won last July’s snap election, it was unable to attract enough support to oust the Sánchez government, even with the backing of the far-right Vox party.

Sánchez’s Spanish Socialist Workers’ party (PSOE), which finished second, managed to form a new minority coalition government after securing the support of the two main Catalan pro-independence parties. It did this by offering an amnesty for those involved in the unilateral effort to secede from Spain that culminated in the illegal independence referendum of October 2017.

The draft law will apply to about 400 people involved in the symbolic independence referendum of November 2014 and the poll that followed three years later, which led to a unilateral declaration of regional independence that plunged Spain into its worst political crisis for four decades.

Its most high-profile beneficiary would be the former Catalan regional president Carles Puigdemont, who fled to Belgium to avoid arrest over his role in masterminding the illegal push to secede.

While the bill has now passed the lower house, it will face tougher scrutiny when it goes before the PP-controlled senate before returning to congress for final approval. Once law, the amnesty will be applied by judges on a case-by-case basis.

The legislation has proved deeply unpopular with many Spaniards. A poll in mid-September showed that 70% of voters, including 59% of the people who voted for the PSOE last July, opposed the measure. The issue has brought hundreds of thousands of people out on to the streets to protest in recent months.

Sánchez argues that the amnesty, which he previously opposed, is needed to help Spain move on from the confrontations of the past. He has also said his decision to pardon nine convicted Catalan independence leaders three years ago helped ease tensions and promote dialogue.

On Thursday he described the law as “a brave and necessary step towards reunion”, saying it would usher in “a new period of coexistence and prosperity in Catalonia”.

But his opponents have accused him of hypocrisy, cynicism and putting his own political survival before the country’s interests.

The PP leader, Alberto Núñez Feijóo, has previously described the amnesty law as “a shameful and humiliating deal” and “the greatest affront to dignity, equality and the separation of powers seen in a western democracy”. Speaking before the vote, he said the law was “not reconciliation, it’s submission”.

Feijóo’s fierce indignation was undermined last month, however, after it emerged that he was in favour of giving Puigdemont a conditional pardon.

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Vox’s leader, Santiago Abascal, said the vote had served to create “a privileged caste of unpunishable politicians” and to humiliate Spaniards by “destroying the condition of citizen equality before the law”.

Aragonès, meanwhile, said the approval of the bill was the culmination of a collective effort that “must put an end to the repression against independence”, adding it would open “a new stage to achieving the freedom of Catalonia”.

A previous version of the amnesty law was rejected by congress at the end of January after Puigdemont’s centre-right Junts party voted against, arguing it did not offer sufficient legal protection for those being investigated for terrorism-related crimes during the push for independence. The bill has since been amended to Junts’s satisfaction.

The already complicated political landscape became knottier still on Wednesday when Aragonès called the 12 May election after accusing his opponents of behaving irresponsibly by voting down his budget.

Although Aragonès’s moderate pro-independence party, the Catalan Republican Left (ERC), was formerly in coalition with the more hardline Junts, the two parties are deeply divided on how best to achieve their shared goal of a sovereign Catalonia.

Junts is hoping that the amnesty will allow Puigdemont, who has been in self-imposed exile for almost seven years, to return to Spain to run as its candidate and help recover the independence movement’s stalled momentum.

While the ERC and Junts took 51% of the vote between them at the last regional election in 2021, the Catalan Socialist party – the regional branch of the PSOE – finished first, winning 23% of the vote.

Recent polls suggest the appetite for an independent Catalonia continues to wane. At the height of the crisis in October 2017, a survey by the Catalan government’s Centre for Opinion Studies found that 48.7% of Catalans supported independence and 43.6% did not. A poll late last year from the same centre found 52% were against and 41% in favour.