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A group representing victims of Spanish dictator Francisco Franco's regime has asked prosecutors to investigate a song performed at a rally by the far-right Vox party for allegedly calling for a return to civil war.

The song, written by youtuber Isaac Parejo and performed with the band Los Meconios, starts with the line "We're going back to '36", a reference to the year Spain's civil war began and a suggestion Spain's Association for the Recovery of Historical Memory said could be akin to a hate crime.

The war started in July 1936 when Franco and other rebel officers launched an unsuccessful coup against the elected government of the Second Republic that escalated into a three-year conflict with about 500,000 casualties.

"The Attorney General's office should investigate and the Secretary of State for Democratic Memory should file a hate crime complaint," the association tweeted in reference to a video of the concert at Vox's annual "Viva" event.

The state prosecutor's office and the ministry in charge of democratic memory declined to comment. The lyrics include verses such as "we are 'fachas'" - a slang term for "fascist" - and comparisons between Spain's current centre-left coalition government and the Popular Front that held power in the lead-up to the civil war.

The performers said on social media the song was a parody and had been misinterpreted. "That sentence ... doesn't mean that we want to go back to '36, it means that they want to take us back to '36," Parejo said in a video on Twitter, referring to what he described as the "single-brain-cell Left".

Several politicians linked to right-wing populist movements sent video messages supporting Vox at the event, including former U.S. President Donald Trump, Hungarian Prime Minister Viktor Orban and Italy's Giorgia Meloni.

Since its founding in 2013, Vox has grown to become the third largest party in the lower house, currently holding 52 of 350 seats. It is also a junior partner in the regional government of Castille and Leon led by the conservative Partido Popular.