The Spanish prime minister has outlined plans to remove Catalonia's leaders and take control of the separatist region.
Speaking after an emergency cabinet meeting on Saturday, Mariano Rajoy stopped short of dissolving the region's parliament but put forward plans for elections.
The measures must now be approved by Spain's Senate in the next few days.
The Speaker of the Catalan Parliament, Carme Forcadell, called the measures a "de facto coup d'etat".
"It is an authoritarian coup inside a member state of the European Union," she said, adding that Mr Rajoy intended to "put an end to a democratically elected government".
Large crowds have gathered in Barcelona to protest against direct rule from Madrid.
It comes almost three weeks after Catalonia held a disputed independence referendum.
Spain's supreme court had declared the vote illegal and said it violated the constitution, which describes the country as indivisible.
Catalan leader Carles Puigdemont has ignored pleas from the national government to abandon moves towards independence.
Mr Rajoy said the the Catalan government's actions were "contrary to the law and seeking confrontation". He said it was "not our wish, it was not our intention" to impose direct rule.
This will be via Article 155 of Spain's constitution, which allows it to impose direct rule in a crisis on any of the country's semi-autonomous regions.
Spanish law dictates that elections must be held within six months of Article 155 being triggered, but Mr Rajoy said it was imperative that the vote be held much sooner.
Reports say that Spain's interior ministry is preparing take control of Catalonia's Mossos police force and remove its commander Josep Lluís Trapero, who is already facing sedition charges.
The government is also considering taking control of Catalonia's public broadcaster TV3, El País newspaper reported.
How have Catalans reacted?
As well as calling Mr Rajoy's actions a coup, Speaker Ms Forcadell said they were not permitted under Article 155. "We are committed to working tirelessly to represent what Catalans voted for," she said.
Catalan Vice-President Oriol Junqueras said Mr Rajoy and his allies had "not just suspended autonomy. They have suspended democracy".
Barcelona Mayor Ada Colau said it was a "serious attack on the rights and freedoms of all, both here and elsewhere" and called for demonstrations.
The president of Barcelona football club, Josep Maria Bartomeu, said the club gave its "absolute support for the democratic institutions of Catalonia chosen by its people".
But he called for any reaction to be "civil and peaceful" and said dialogue was the only way to a solution.
Eduard Rivas Mateo, spokesman for the Catalan Socialist party - which supports the Spanish government's stance but also wants constitutional reform - said he could not accept a "harsh application" of Article 155.
But Inés Arrimadas, head of the centrist Ciudadanos party in Catalonia, which is against independence, said holding fresh elections would "restore goodwill and democracy" in the region.
Catalan leader Mr Puigdemont is due to speak at 21:00 local time (19:00 GMT).
Solution or provocation?
Guy Hedgecoe, BBC News, Madrid
Mariano Rajoy's use of Article 155 had been widely anticipated, but his announcement when it came still had a huge impact. The article has never been invoked before, so there was a certain amount of mystery surrounding its potential reach and meaning.
Although Mr Rajoy insisted that Catalonia's self-government is not being suspended, many will disagree. The removal from office of Carles Puigdemont and all the members of his cabinet, to allow ministers in Madrid to take on their duties, amounts to a major reining in of Catalonia's devolved powers.
The Spanish prime minister said that one of his aims is to restore peaceful co-existence to Catalonia with these measures. Many Catalans who want to remain in Spain will approve of this strident action. But those who want independence for their region are likely to see this as a provocation rather than a solution.
How did we get here?
Catalonia's regional government held a referendum on 1 October to ask residents of the region if they wanted to break away from Spain.
Of the 43% of Catalans said to have taken part, 90% voted in favour of independence. But many anti-independence supporters boycotted the ballot, arguing it was not valid.
Mr Puigdemont and other regional leaders then signed a declaration of independence, but immediately suspended it in order to allow for talks.
He then defied two deadlines set by the national government to clarify Catalonia's position.
Are you in Catalonia? Share your views and experiences by emailing email@example.com.
Please include a contact number if you are willing to speak to a BBC journalist. You can also contact us in the following ways: