Barcelona, Spain (CNN) Catalan's secessionist leaders appear determined to defy the King of Spain and go ahead with a unilateral declaration of independence -- a move that would steer the country's deepening constitutional crisis into uncharted waters.

The office of the Catalan President Carles Puigdemont said he would appear before the regional parliament on Monday, despite a warning from the monarch that Catalan leaders had acted "outside the law" of Spain by holding a banned referendum on independence.

Puigdemont, who told the BBC that an independence declaration would come within days, is due to address Catalans in a TV broadcast from Barcelona on Wednesday evening.

Spanish FM: No excessive use of force in Catalonia

Spanish FM: No excessive use of force in Catalonia 05:19

Spanish FM: No excessive use of force in Catalonia

The Catalan police force, Mossos, told CNN that Spain's high court had summoned highest-ranking officer to answer accusations of sedition -- provoking a rebellion against the state. Spanish authorities believe local police did not do enough to prevent Sunday's banned independence vote from taking place.

Both sides appeared set on a collision course after the referendum went ahead in the face of a violent police crackdown

King Felipe's rare TV speech was unexpectedly hardline -- he accused pro-independence leaders of "unacceptable disloyalty" and made no mention of the nearly 900 people injured in clashes with Spain's national security forces.

Instead, the monarch blamed the referendum's organizers for the strife.

In a BBC interview recorded before the King's statement, Puigdemont said his government would "act at the end of this week or the beginning of next" to split from Spain.

A unilateral declaration of independence would severely test the government of Prime Minister Mariano Rajoy. If Madrid decides that Catalonia is acting unconstitutionally, it could invoke emergency powers to take control of the Catalan government.

Protesters gather at the Placa de la Universitat square in Barcelona in Tuesday's day of protest against police violence.

The sight of Spanish forces seizing Catalan institutions would further polarize opinion in the region, reeling from Sunday's crackdown. Barcelona's city police said 700,000 people took part Tuesday in a day of protest against the police violence.

Spain's Foreign Minister denied that security forces used excessive force on Sunday. "If there was any use of force by police in any way it was because they were prevented from doing what they were asked to do," Alfonso Dastis told CNN's Christiane Amanpour.

King's hardline speech

In his TV address, King Felipe called the situation "extremely serious" and said the pro-independence camp had demonstrated "an unacceptable disloyalty towards the powers of the state -- a state that represents Catalan interests."

The "irresponsible attitude" of the regional government has "put the economic and social stability of Catalonia and Spain at risk," he said.

The King of Spain delivers his TV address Tuesday night.

The King said Catalan authorities had acted "outside the law" and emphasized the crown's firm support for the constitution, reiterating "commitment as King to the unity and permanence of Spain."

The King's address made it clear that he supported the stance of Rajoy, who has refused to entertain dialogue with the Catalan authorities.

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Rajoy has the option of imposing direct rule on Catalonia under Article 155 of the Spanish constitution, drawn up after the Franco military dictatorship was toppled in 1975. Regions like Catalonia and the Basque country were given sweeping freedoms and control, but the constitution also protected the integrity of Spain and gave Madrid powers to seize back control if regions acted beyond the law.

Such a move would be a last resort. Puigdemont told the BBC it would be "an error which changes everything."

Pablo Guillen Alvarez, an economist and ‎associate professor at the University of Sydney, said article 155 gave Madrid wide powers. "The central government can run the police, the schools, hospitals in lieu of the Catalan government, and the Catalan government couldn't legally do any thing against it," he said.

Independence risk

Declaring independence would be a huge gamble for Puigdemont. While there was broad support for holding the referendum, support for independence is not overwhelming in Catalonia.

Catalan authorities said 90% of voters in Sunday's referendum backed a split from Madrid -- but turnout was only 42%. The Catalan government blamed the crackdown for the low turnout and said up to 770,000 votes were lost as a result of raids at polling stations.

Taxi driver Ana Maria Lopez is among the Catalans who oppose independence for Catalonia, telling CNN that Puigdemont "thinks he's a king." She added: "Him declaring independence is like me declaring 'From tomorrow I'm a princess.'"

At the same time, she was critical of the central government in Madrid, saying its poor handling of the situation had made things worse.

Alvarez said an independence declaration was "tempting" for Puigdemont. "People in the streets are calling for it ... and the central government seems unrepentant, committed to cracking down on the pro-independence movement."

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He suggested Puigdemont may be trying to gain leverage from the central government. The Catalan government wants the European Union to intervene, but Madrid has rejected any suggestion of outside mediation, saying it was an internal matter.

Speaking at a debate on Catalonia in the European Parliament on Wednesday, European Commission Vice-President Frans Timmermans lamented Sunday's events. "Violence does not solve anything in politics," he said.

But he backed the Spanish government's stance. "This is an internal matter for Spain that has to be dealt with in line with the constitutional order of Spain," he said.

At the same time, Timmermans urged the sides to "move from confrontation to dialogue," adding: "All lines of communication must stay open. It's time to talk to find a way out of the impasse."