Spain's socialist Prime Minister Pedro Sanchez has said he will not step down from his post amid a corruption investigation against his wife.

He said Monday that he will continue in office "even with more strength" after days of reflection over what he has denounced as a campaign of political harassment by the right.

"I have decided to stay," he said in a highly-anticipated public address that drew a line under days of political uncertainty that had gripped the country for the past five days.

In office since 2018, the 52-year-old Socialist leader had on Wednesday written a letter to the public saying he was taking time out to mull his possible resignation after a Madrid court confirmed a preliminary probe into his wife Begona Gomez for suspected influence peddling and corruption.

Denying the move was a "political calculation," Sanchez said he needed "to stop and reflect" on the growing polarization within politics which he said was increasingly being driven by "deliberate disinformation."

"For too long we've let this filth corrupt our political and public life with toxic methods that were unimaginable just a few years ago ... Do we really want this for Spain?" he asked.

"I have acted out of a clear conviction: either we say 'enough is enough' or this degradation of public life will define our future and condemn us as a country."

He said his decision to stay on had been "decisively influenced" by the mass show of support outside the Madrid headquarters of his Socialist party, where thousands of emotional supporters had chanted: "Pedro, stay!"

The public prosecutor's office Thursday asked that the investigation into Begona Gomez be closed but Sanchez an expert in political survival who has made a career out of taking political gambles, held his silence.

He had been due to launch his party's campaign Thursday for the May 12 Catalonia regional elections in which his Socialists are hoping to oust the pro-independence forces from power.

'Harassment' campaign

The court opened its investigation into Sanchez's wife in response to a complaint by the anti-corruption pressure group Manos Limpias (Clean Hands), whose leader is linked to the far-right.

Shortly after Sanchez's bombshell letter went out on X, the group, which has presented a litany of unsuccessful lawsuits against politicians in the past, said it had based its complaint on media reports and could not vouch for their veracity.

While the court did not give details of the case, online news site El Confidencial said it was related to her ties to several private companies that received government funding or won public contracts.

Right-wing opponents and media have vilified Sanchez because his minority government relies on the support of the hard left and Catalan and Basque separatist parties to pass laws.

They have been especially angered by his decision to grant an amnesty to hundreds of Catalan separatists facing legal action over their roles in the northeastern region's failed push for independence in 2017.

That amnesty, in exchange for the support of Catalan separatist parties, still needs final approval in parliament.

The opposition has since Wednesday mocked Sanchez's decision to withdraw from his public duties as an attempt to rally his supporters.

"A head of government can't make a show of himself like a teenager and have everyone running after him, begging him not to leave and not to get angry," said right-wing opposition leader and Popular Party head Alberto Nunez Feijoo on Thursday.