Any declaration of independence by Catalonia will have no effect, Spanish Prime Minister Marian Rajoy has warned, adding that he is not ruling out suspending the region’s autonomy.
In an interview with El Pais newspaper, Mr Rajoy also rejected any mediation to resolve the crisis.
Earlier, thousands of people rallied across the country for Spanish unity.
The demonstrations were in response to last Sunday’s disputed referendum on Catalan independence.
The final results from the wealthy north-eastern region showed 90% of the 2.3 million people who voted backed independence. Turnout was 43%.
There have been several claims of irregularities, and many ballot boxes were seized by Spanish police.
Nearly 900 people were injured as the police, trying to enforce a Spanish court ban on the vote, attempted to disperse voters.
Thirty-three police officers were also hurt.
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In the El Pais interview, Mr Rajoy said: “The government will ensure that any declaration of independence will lead to nothing.”
Asked whether he was prepared to invoke Article 155 of Spain’s constitution, which allows the national parliament to intervene in the running of an autonomous region, Mr Rajoy said: “I don’t rule out absolutely anything that is within the law.”
The prime minister also said he planned to keep extra police deployed to Catalonia before the referendum until the crisis was over.
And he added that he would not call early national elections because of the growing political crisis.
Catalan President Carles Puigdemont is expected to address the regional parliament on Tuesday at 18:00 local time (16:00 GMT) after Spain’s Constitutional Court earlier suspended the Catalan parliament session that had been planned for Monday.
There is speculation that the parliament will declare independence unilaterally at its next sitting.
Meanwhile, the former leader of Catalonia, Artur Mas, told the Financial Times newspaper that the region was not yet ready for real independence – even though he believed it had won the right to break away.
On Saturday, thousands of people calling for Spanish unity attended rallies in the capital Madrid.
Other demonstrations – including in the Catalan city Barcelona – have also been held urging political dialogue.
Protesters dressed in white gathered with signs saying “Spain is better than its leaders” and “Let’s talk”.
Meanwhile, businesses have continued to announce their departure from the Catalan region amid the ongoing political uncertainty.
More on the Catalan crisis
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Catalan independence declaration ‘will have no effect’