Tens of thousands of Spaniards protested in Madrid against possible plans by acting Prime Minister Pedro Sanchez to grant an amnesty to Catalan separatists to keep himself in office after an election he failed to win.

Waving Spanish flags, supporters of the opposition conservative People's Party (PP) travelled from across Spain to attend the rally in Madrid. Authorities estimated the size of the crowd at 40,000.

Sanchez, who came second in an election in July, could stay in office if he wins the support of exiled former Catalonia leader Carles Puigdemont, whose Junts per Catalunya party controls seven seats in parliament.

Puigdemont, wanted in Spain for attempting the region's secession, has demanded that legal action be dropped against fellow separatists as a condition for his support.

Alberto Nunez Feijoo, leader of the PP which won the most votes on July 23, said he would never concede to demands for an amnesty for organisers of a 2017 independence referendum in Catalonia, which was held despite courts ruling it was illegal.

Withdrawing criminal cases against the separatists would amount to granting an amnesty to "coup plotters", he told supporters at the Madrid rally.

Gregorio Casteneda, 72, a pensioner, had travelled from Santander on Spain's north coast to show his opposition to any amnesty.

"I am not in favour of the government that we have. To me this is a disaster because it is going to divide Spain totally," he told Reuters.

Sanchez held his own political rally in Gava, near Catalonia's regional capital Barcelona, on Sunday. He did not mention an amnesty but said the Socialists wanted to heal social divisions over the Catalan crisis.

"We are trying to turn the page," he told supporters.

In 2021, Sanchez granted pardons to nine separatists jailed over their roles in the independence push.

Feijoo will take the first stab at a vote to become prime minister on Sept. 27, but his chances of winning are seen as slim since the PP opposes any concessions to separatists. If Feijoo fails, Sanchez will get a chance to see if he can muster support.