- Video report by ITV News Correspondent Emma Murphy
Catalonia's President has said the autonomous region will declare independence from Spain in a matter of days.
Speaking to the BBC, Carles Puigdemont said his government would "act at the end of this week or the beginning of next", and admitted there was currently no contact between his devolved administration and the government in Madrid.
When asked about the possibility of the Spanish government intervening and taking control of the Catalan government, Mr Puigdemont said it would be "an error which changes everything".
In a speech a short time later, Spain's King Felipe VI said the Catalan authorities have deliberately bent the law with their "irresponsible conduct".
Delivering an address to the nation by television, the king said that the bid by authorities in the northeastern region to push ahead with independence has "undermined coexistence" in Catalonia.
"Today, Catalan society is fractured and confronted," King Felipe said, referring to the political crisis as "very serious moments for our democratic life".
He said that the state needs to ensure Spain's constitutional order and the correct functioning of Catalan institutions and rule of law.
Spain's conservative government has said it will respond with "all necessary measures" to counter the Catalan defiance.
It is holding talks with national opposition leaders to try to find consensus on the response, which could include suspending the region's self-government.
Separatist groups and unions had initially called for strikes to be held on Tuesday in support of Catalan leaders pushing ahead with a declaration of independence from Spain.
But many non-separatists were also drawn to the streets following the crackdown on the referendum vote on Sunday.
The main national unions, CCOO and UGT, rejected the strike but told workers to join protests.
In Barcelona's Catalonia and University squares, a sea of demonstrators waved flags, most of them "esteladas," embraced by those seeking secession, but there were also plenty of Spanish national flags.
Among many banners displayed, one read "Stop violence, #CataloniaIsComing" and another one asked: "Where are you Europe?"
One of the biggest groups concentrated around the Spanish national police headquarters in Barcelona, where protesters called them "occupying forces" and called for Prime Minister Mariano Rajoy to resign.