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.
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.
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.
According to Sky sources, three government yachts with members of the Policia Nacional, the primary anti-riot body in Spain, have docked in Barcelona.
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.
“They are basically occupying the streets with cops,” one Barcelona resident told Sky News.
Barcelona football club has condemned the actions of the police, saying the club supports the right to free expression.
But Spanish Prime Minister Mariano Rajoy 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.
And 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.