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The Spanish government has warned the Catalan authorities that national police could be sent to their region if they tolerate separatist blockades.

The warning - in three letters from ministers - followed a protest on Saturday by separatists who blocked the AP-7 motorway connecting France to Catalonia's coast.

Madrid accused the Catalan police force - the Mossos - of just standing by.

Spain's ruling Socialists oppose the Catalan independence movement.

Catalan nationalists regained power in Barcelona in May, after a seven-month period of direct rule by Madrid.

Tensions remain high, as many Catalans resent Madrid's show of force last year, when it charged pro-independence leaders with sedition. Nine are in Spanish pre-trial detention, four of them now into the second week of a hunger strike.

The hunger strikers - Jordi Sanchez, Jordi Turull, Josep Rull and Joaquim Forn - accuse Spanish courts of deliberately delaying their appeals.

Pro-independence groups called Committees to Defend the Republic (CDR) blocked the AP-7 for 15 hours, in solidarity with the jailed Catalan separatist leaders.

On Sunday CDR activists also lifted toll barriers on the motorway, enabling motorists to pass through toll-free. It was a particularly busy long weekend, including two public holidays.

The letter from Spanish Interior Minister Fernando Grande-Marlaska noted a state security law which would allow Madrid to send police to Catalonia if the Mossos neglected their duties.

The law "provides for action by the state security forces when the state authorities deem it expedient", he told his Catalan counterpart.

The Madrid government, he said, "requires the Mossos to fulfil their legal duties".

"If that does not happen, an intervention by state security forces will be ordered."

After many huge nationalist demonstrations in Barcelona there is much nervousness about a planned meeting there between Spanish and Catalan ministers on 21 December. Spain will send a security contingent to protect its ministers.

Spanish Prime Minister Pedro Sánchez heads a minority government that depends on nationalists - including Catalans - to stay in power, but he has ruled out any new Catalan referendum on independence.

In an election on 2 December his Socialists lost 14 seats in Andalusia - long a Socialist stronghold. There were big gains for right-wing parties strongly opposed to Catalan separatism - Vox and Ciudadanos.

In a further ratcheting-up of tension, Catalan premier Quim Torra irritated the Spanish government by praising Slovenia's successful - but bloody - path to independence. It broke away from Yugoslavia in 1991.

After visiting Slovenia, Mr Torra said "Slovenians decided to forge ahead no matter what the consequences.

"Let us be like them, and let us be ready for everything in order to live free."

Spanish Foreign Minister Josep Borrell said "it is the kind of language that seems to be calling for an insurrection".