Tens of thousands of people gathered in the Spanish capital Sunday to call for fresh elections and denounce the incumbent prime minister's handling of the ongoing political and territorial crisis with local authorities in Spain's northeastern Catalonia region.

It was one of the first major demonstrations against Socialist Party leader Pedro Sánchez since he took office eight months ago following a no-confidence motion against a conservative executive. According to police figures, an estimated 45,000 people, many waving national flags, heeded the call of a loose coalition of center-right and right-wing parties to congregate in downtown Madrid under the banner: "For a united Spain. Elections now!"

"Thousands of Spaniards are saying on this Sunday no to separatism and yes to Spain," Albert Rivera, leader of the center-right, business-friendly Ciudadanos said in a speech to press given on the sidelines of the demonstration. "That's enough of separatists defining our journey and enough of the Spanish population forced to be at the service of those who want to liquidate this country," he added.

The demonstration took place in Plaza de Colón (Columbus Square) in central Madrid, which is home to the largest Spanish flag in the world. Organizers claimed that 200,000 people attended the rally, quadrupling police estimates.

Calls for the protest grew after Sánchez's minority executive outlined a proposal to appoint a mediator to facilitate dialogue with the regional Catalan government in a bid to pass the government's budget.

While the Socialist Party opposes Catalan independence, Sánchez only rose to power thanks to the votes of the Catalan European Democratic Party (PDeCat) and the Republican Left of Catalonia (ERC) party, whose lawmakers backed him in the successful no-confidence vote against former PM Mariano Rajoy (PP).

The Spanish government now desperately needs those same votes from ERC and PDeCAT, who now jointly govern Catalonia, in order to pass an ambitious budget bill that would be doomed without the Catalan nationalists' support.

Rivera said he was proud of taking part in the protest and warned Sánchez that “if he continues to let pro-independence campaigners negotiate privileges and pardons he will have us to contend with.”

The demonstration kicked off with a reading of a joint statement on the unity of Spain delivered by three journalists.

Joining the two main center-right parties were far-right formation Vox, headed by Santiago Abascal, which recently made significant gains in recent regional elections in Andalusia, and four regional parties, Navarrese People’s Union, the Asturias Forum, the Aragonese Party and the Union Progress and Democracy Party, who also rallied their supporters for the nationalist protest.

“Seven political forces who say enough blackmail and enough subservience,” the PP's leader, Casado told reporters at the event. "We say enough is enough; call elections now. It is us Spaniards who must decide what Spain is and it is this great nation that we have built over the last 40 years," he added.

Speaking from the northern city of Santander, where he was attending an event, Sánchez said that he respected the demonstration but defended his government's actions.

"What I'm doing now as prime minister is resolving the national crisis that the PP aggravated when it was in government," he said.

The Catalan crisis, which had been simmering for years, exploded in 2017 with the events that led up to the Oct. 1 referendum on the prosperous region's secession from the rest of the country, which was deemed unconstitutional by the judiciary. Catalans, according to the results of the illegal ballot, overwhelmingly chose to become independent from Spain.

Several days after, the regional government unilaterally declared independence and Spain triggered a constitutional mechanism to strip the region of its autonomy, depose the local government and dissolve parliament.

Fresh elections saw the pro-independence parties again eke out a slim majority in the regional parliament, thus assuring a continuation of the territorial conflict between Barcelona and Madrid.

Sunday's protests in Madrid came just days before 12 jailed Catalan politicians and activists leaders are to go on trial at the Supreme Court for their alleged involvement in the referendum and independence declaration, another issue that has fueled schisms between Madrid and Barcelona.

They could face charges of sedition, rebellion and misuse of public funds.

The former Catalan president Carles Puigdemont remained in self-exile in Belgium but Spanish authorities have in place a warrant for his arrest to face the same charges.