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MADRID (AP) — Thousands in Madrid are protesting the Spanish government’s plan to issue pardons to a dozen Catalan separatist leaders convicted for their roles in the biggest challenge to the country’s unity in recent history.

The demonstration has been organized by a civil society group in defense of the nation’s unity, and it chose a central square that has become a symbol for far-right political rallies.

Leaders of the center to far-right opposition to Socialist Prime Minister Pedro Sánchez are joining the protest at Colón Square, which boasts one of the nation’s largest national flags.

Sánchez hasn’t announced pardons for the 12 political and civil society leaders who in October 2017 pushed ahead with a banned referendum on independence for the northeastern region of Catalonia and then declared independence based on its results. But he has defended the possible move as a way to bring Catalans and Spaniards closer together after the divisive convictions that put most of the Catalan leaders behind bars.

The prime minister is also facing criticism in his own Socialist camp, where the move is seen as a risky political gamble. While more than 60% of Spaniards oppose the pardons and only 29.5% back them, according to a recent poll for El Mundo newspaper, surveys in the northeastern region show support for the move ranging in between 60% to 70% of Catalans.

Detractors say the separatists have not shown any remorse for their defiance to the Spanish Constitution and that Sánchez is making concessions to them in exchange for support in the national parliament.

Tensions over secession grew in earnest a decade ago amid the economic hardship of the Great Recession and discontent over Spain’s opposition to more autonomy for the Catalan-speaking region of 7.5 million people.

The issue has also dominated the political debate at the national level and contributed to the rise of the Spanish nationalist Vox party, which has become the third-largest political force in the Spanish Congress.