Spain is set to put in place measures to take direct control of Catalonia in response to the region's declaration of independence last week.
On Friday, Madrid stripped Catalonia of its autonomy and removed Catalan leader Carles Puigdemont from office.
The temporary move will see as many as 150 of the region's ministers replaced. Some have vowed to continue to work.
Mr Puigdemont and other Catalan officials may face criminal charges, a move likely to lead to huge protests.
What happens next?
Spain's central government is to take control of Catalan institutions with Spanish officials expected to be put in place in the region's ministries on Monday.
Mr Puigdemont and his vice-president Oriol Junqueras have said they do not accept the move by Madrid, adding that they could only be removed from power by the citizens of Catalonia.
If Mr Puigdemont and others refuse to step aside, they face possible arrest.
Catalonia's regional police force, known as Mossos, whose chief was dismissed last week, are deployed in Sant Jaume square, near the government palace in the centre of Barcelona.
Spain's Foreign Minister Alfonso Dastis has said he expects the officers to "act professionally".
Spain's chief prosecutor has also been preparing criminal charges against any officials considered to have acted against Spanish law in declaring independence following a referendum deemed illegal under the Spanish constitution.
Meanwhile, Madrid has called for fresh regional elections on 21 December.
Mr Puigdemont could run in new elections, according to Mr Dastis, but only if the sacked Catalan leader has not been jailed.
Madrid's enforcer for Catalonia
What next for Spain?
What about Catalonia's autonomy?
On Sunday, Mr Dastis told Sky News: "We are not taking autonomy away from Catalonia. We are just re-establishing it, in fact."
He added: "Reality is already sinking in, will continue sinking in and they will realise that they cannot do something without the authority of law and they will be usurping authority."
Meanwhile, Interior Minister Juan Ignacio Zoido has written to all police officers in Catalonia asking for their loyalty as a "new era" begins in the region.
He reminded members of the regional police, who are now under direct control from Madrid, of their duty to obey orders and to guarantee "the rights and liberties of all".
Senior police officers have told the BBC that they have already complied with an order to remove framed photographs of Mr Puigdemont from police stations across the region.
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