Pro-independence supporters have gathered outside Barcelona's top court for a second consecutive day, protesting against the Spanish government's attempts to block a referendum on Catalonia's split from Spain.

The demonstrations began after Spanish authorities, which outlawed the vote scheduled for October 1, detained on Wednesday 14 regional officials and seized referendum materials, including some 10 million ballot papers, in an unprecedented raid of regional government offices.

The protesters on Thursday packed the Barcelona boulevard connecting Arc de Triomf and Parc Ciutadella, two popular tourist attractions, waving signs reading "Stop dictatorship" and "We want to vote".

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"Certainly, the people here are saying they are not going anywhere," Al Jazeera's Karl Penhaul, reporting from the protest site in Catalonia's main city, said.

"They have been chanting slogans such as, 'The streets are ours', a clear determination that they think that one of the ways they can force through this referendum, despite the growing police action, is to try and mobilise, get out there and maintain a presence in these public spaces."

Among those detained was Josep Maria Jove, Catalonia's junior economy minister.

Penhaul said that two of the detained were released overnight, but the other 12 were "still facing possible charges of trying to organise and facilitate a referendum that the state prosecutors are calling illegal".

Acting under court orders, police also raided printers, newspaper offices and private delivery companies in a search for campaign literature, instruction manuals for manning voting stations and ballot boxes.

On Thursday, Catalan leaders acknowledged for the first time that plans to hold the referendum were now in doubt.

"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," he said, adding, however, that he was convinced voters would still turn out in large numbers.

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On Wednesday, Catalonia's President Carles Puigdemont said the government's actions imposed a "de facto" state of emergency and denounced the "totalitarian and undemocratic attitude of the Spanish state".

Later in the day, Prime Minister Mariano Rajoy denounced the referendum as undemocratic and called for Catalonia's pro-independence leaders to "stop this escalation of radicalism and disobedience once and for all" in a televised statement.

Spain's constitution grants the central government exclusive power to hold referendums.

Polls show about 40 percent of Catalans support independence although a majority want a referendum on the issue.