BARCELONA, Spain -- Spanish unionists in Catalonia finally found their voice on Sunday, resurrecting Spain's flag as a symbol of patriotism after decades of it being associated with the Franco dictatorship.

In a defiant challenge to plans by Catalonia's regional government to unilaterally declare independence, hundreds of thousands of people flooded the streets of Barcelona in a surprising outpouring of Spanish unity.

Spanish Prime Minister Mariano Rajoy has said any declaration of independence would be seen as unconstitutional and they would take any legal measures possible to stop that independence, CBS News' Seth Doane reports from Barcelona

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The crowds chanted "we are the silent majority," Doane reports. He said the people said they did not go to the polls either because they were not from the region or they did not believe the vote was legal. He said they feel their voice was not heard.

Peruvian literature Nobel Laureate Mario Vargas Llosa addresses a pro-union demonstration organised by the Catalan Civil Society organisation in Barcelona, Spain on Oct. 8, 2017. GONZALO FUENTES / REUTERS

They chanted "Don't be fooled, Catalonia is Spain" and called for regional president Carles Puigdemont to go to prison for holding an illegal referendum last week. Some of the demonstrators took to rooftops, including families with children, and leaned over ledges from their perches overlooking the streets below to wave giant Spanish flags in a city accustomed to the prevalence of the Catalan pro-independence "estelada."

Spain's red-and-yellow flag has long been taboo here in Catalonia and throughout the country because it has been linked to groups supportive of Gen. Francisco Franco's dictatorship. But on Sunday, a sea of Spanish flags, interspersed with some Catalan and European Union flags, dominated Barcelona's boulevards.

Barcelona police said 350,000 people participated, while march organizers Societat Civil Catalana said that 930,000 people turned out. The march was peaceful and no major incidents were reported.

Puigdemont has pledged to push ahead for independence and is set to address the regional parliament on Tuesday "to report on the current political situation." In the days after the Oct. 1 referendum, the momentum appeared to be on his side. Pro-independence protests were attracting large numbers and he benefited politically from a violent crackdown by Spanish police during the referendum voting.

But now the tide seems to be turning. Catalonia's top two banks announced they were relocating their headquarters to other parts of Spain because of financial uncertainty if there is an independence declaration. Other companies are reportedly considering leaving Catalonia to avoid being cast out of the EU and its common market in the case of secession.

And Sunday's mass demonstration by pro-unity Catalans, under the slogan of "Let's recover our common sense!" will put further pressure on Puigdemont. The march was the largest pro-unionist showing since the rise of separatist sentiment in the prosperous northeastern region that has pushed Spain to the brink of a national crisis.

The rally comes a week after the Catalan government went ahead and held a referendum on secession that Spain's top court had suspended and the Spanish government said was illegal.

Catalan authorities say the "Yes" side won the referendum with 90 percent of the vote, though only 43 percent of the region's 5.3 million eligible voters turned out in polling that was marred by police raids of polling stations on orders to confiscate ballot boxes.

In an interview with the Spanish newspaper El Pais published Sunday, Rajoy said that he will consider employing any measure "allowed by the law" to stop the region's separatists. Catalonia represents a fifth of Spain's economy.

Rajoy said that includes the application of Article 155 of the Spanish Constitution, which would allow the central government to take control of the governance of a region "if the regional government does not comply with the obligations of the Constitution."

"The ideal situation would be that I don't have to find drastic solutions, but for that to happen there will have to be some rectifications (by Catalan leaders)," Rajoy said.

Rallies were held Saturday in Madrid, Barcelona and other cities to demand that Rajoy and Puigdemont negotiate to find a solution to Spain's worst political crisis in nearly four decades.

"I hope that nothing will happen. Because (Catalonia) is going to lose more than (Spain) because businesses are fleeing from here already," said protester Juliana Prats, a Barcelona resident. "I hope it will remain like it has been up until now, 40 years of peace."

The rally drew Spaniards from outside the northeastern region to the Catalan capital. One group held a large banner boasting "Marbella," a town on Spain's southern coast. An AP reporter spoke with another man who had come from the northern Basque Country region.

Nobel Literature Prize laureate Mario Vargas Llosa and former president of the European Parliament Josep Borrell addressed the rally.

"Besides Catalans, there are thousands of men and women from all corners of Spain who have come to tell their Catalan companions that they are not alone," said Llosa, who took on Spanish citizenship in addition to that of his native Peru in 1993. "We want Barcelona to once again be the capital of Spanish culture."

Borrell added that: "Catalonia is not a state like Kosovo where rights were systematically violated."

The most recent polls taken before the referendum showed that Catalonia's 7.5 million residents were roughly split over secession, while a majority would support an official referendum on independence if it were condoned by Spanish authorities

Rajoy's government has repeatedly refused to grant Catalonia permission to hold a referendum on grounds that it is unconstitutional since it would only poll a portion of Spain's 46 million residents.

Catalonia's separatists camp has grown in recent years, strengthened by Spain's recent economic crisis and by Madrid's rejection of attempts to increase self-rule in the region.