Thousands of people protested in Madrid on Saturday against Prime Minister Pedro Sanchez's leftist government in a rally that was backed by the far-right party Vox.

Participants waved red and yellow Spanish flags and called on Sanchez to resign. Some held up signs with a photo of the Socialist premier calling him a "traitor".

Around 30,000 people gathered in Madrid's Cibeles Square, according to the central government's delegation in the Spanish capital. Organisers said some 700,000 people had taken part.

The protest was called by dozens of mainly right-leaning civil society groups and backed by conservative parties including the main opposition conservative Popular Party (PP) and Vox.

The right is angered by the government's decision to abolish the crime of sedition, of which nine separatist leaders were convicted over their role in the Catalonia region's abortive secession bid in 2017.

It was replaced with an offence carrying a shorter prison sentence.

Conservatives are also angered by a flagship law against sexual violence that toughened penalties for rape but eased sentences for other sexual crimes.

This has set some convicts free after their jail terms were reduced.

Speaking to reporters at the start of the rally, Vox leader Santiago Abascal denounced "the worst government in history" which "has divided Spaniards and freed rapists and coup leaders".

"We need a permanent and massive mobilisation until the autocrat Pedro Sanchez is expelled from power," he added.

In 2021, Sanchez pardoned Catalan separatists initially sentenced to between nine and 13 years in prison for their role in a failed secession bid.

Catalonia's attempt to become an independent state sparked Spain's worst political crisis in decades, with the then-Catalan leader Carles Puigdemont and several others fleeing abroad to escape prosecution.

PP leader Alberto Nunez Feijoo, who has tried to push the formation to the centre since becoming its leader in April, was not at the rally but encouraged party supporters to attend.

Most polls suggest the PP would win a general election expected at the end of the year but would need the support of Vox to govern.

Before that, Spain will vote in May in regional and local elections.

One of the main dilemmas facing Feijoo is whether to continue pursuing political alliances with Vox as it has in some region or to freeze them out to try to widen the PP's base.

Vox splintered off from the PP in 2013 and is now the third-largest force in parliament. (AFP)