Catalan leaders have acknowledged that plans to hold a referendum on independence from Spain on 1 October are in doubt following the arrest of senior regional officials and the seizure of campaign material by national police.
The Spanish government, facing one of the country's biggest political crises since the end of the Franco dictatorship and return of democracy four decades ago, has called the referendum an illegal act and taken police and court action to block it.
Acting under court orders, police have also raided printers, newspaper offices and private delivery companies in a search for campaign literature, instruction manuals for manning voting stations and ballot boxes.
"It is obvious that we won't be able to vote as we would have liked," Oriol Junqueras, deputy head and economy minister of the regional government, told local television TV3.
"They have altered the rules."
It was the first time the promoters of the referendum acknowledged their plans were in doubt, although Mr Junqueras said he said he was convinced voters would still turn out in numbers.
It is not yet clear whether the police operation would be enough to prevent the vote overall or if it could instead bring fresh momentum to the secession campaign.
Polls show about 40% of Catalans support independence, although a majority want a referendum on the issue.
Following the raids by the Guardia Civil, tens of thousands of protesters gathered outside the regional government offices in central Barcelona as well as in several Catalan cities, waving the red-and-yellow Catalan flag and chanting "Occupying forces out" and "Where is Europe?"
Several hundred people gathered in front of the High Court of Justice of Catalonia today to demand the release of the dozen officials arrested.
They packed the boulevard connecting Arc de Triomf and Parc Ciutadella, two popular tourist attractions, waving signs reading "Stop dictatorship" and "We want to vote."
The central government's spokesman said the protests had been organised by a small group and were not representative of the general feeling of the people.
"In those demonstrations, you see the people who go but you don't see the people who don't go, who are way more and are at home because they don't like what's happening," Inigo Mendez de Vigo said.
Mr Mendez de Vigo also said an offer for dialogue from Madrid remained on the table. Repeated attempts to open negotiations between the two camps over issues such as taxes and infrastructure investment have failed over the past five years.
Prime Minister Mariano Rajoy said yesterday the operations in Catalonia were the result of legal rulings and were to ensure the rule of law.
He called on Catalan leaders to cancel the vote.
"Don't go ahead, you don't have any legitimacy to do it. Go back to the law and democracy. This referendum is a chimera," he said in a televised speech.
But the central government must tread a fine line in enforcing the law in the region without seeming heavy-handed.
Catalan regional president Carles Puigdemont tweeted last night: "We will not accept a return to the darkest times. The Catalan government stands by freedom and democracy" - an allusion to the dictatorship of the Franco era.
