Catalan leaders have described the Spanish state's actions as "totalitarian"
Police have arrested at least 12 people in raids on Catalan government offices, ahead of an independence referendum authorities in Spain are calling illegal.
Catalonia's leader has accused Madrid of imposing a "de facto state of emergency" in the Spanish region and said the arrests were a "shameful attack" on its autonomy.
"We condemn and reject the anti-democratic and totalitarian actions of the Spanish state," Catalan President Carles Puigdemont said in a televised address.
In a statement released online, he argued: "No other member of the European Union is going through what Catalonia is going through.
"To all democrats, in and outside of Catalonia, who today feel indignation about the events of these last few hours, the Government of Catalonia reaffirms its commitment to them and will always ensure their protection.
He added: "We will not accept a return to former times and we will not accept them preventing us from deciding on our free and democratic future.
Among those detained are Josep Maria Jove, the Catalonian secretary general of economic affairs, and Vice President Oriol Junqueras.
The region is seeking a vote to secede from Spain on 1 October.
Thousands of pro-independence protesters took to the streets of Barcelona in the wake of the latest developments, with one group blocking the path of a vehicle belonging to the Spanish police, Guardia Civil.
Recent crackdowns have included forcing internet service providers to block access to websites advertising the vote, and Spanish authorities are seeking to prevent voting cards being posted out.
Hundreds of protesters gathered outside the regional government offices in the centre of Barcelona's tourist district following the arrests.
The Spanish interior ministry has said all time off and vacation will be suspended for Policia Nacional and Guardia Civil members stationed in Catalonia until 5 October.
Barcelona football club has condemned the actions of the police, saying the club supports the right to free expression.
Spanish Prime Minister Mariano Rajoy, however, has described the independence movement as intolerable.
"There's no democratic state in the world that would accept what these people are planning," he said.
Foreign minister Alfonso Dastis told Bloomberg that referendums were the "weapon of choice of dictators".
The Spanish government is backed by the nation's constitutional court in declaring the referendum illegal.
Catalonian secessionists complain that appointments to the court are made by the largest political parties in Madrid, and are therefore not independent.
Opinion polls have shown a near even split in support for & against Catalan independence.