Spanish Prime Minister Mariano Rajoy has gone on national TV to urge Catalan separatists to abandon their plans for a referendum on 1 October.
He asked the region’s government to give up its “escalation of radicalism and disobedience”.
The Spanish authorities have sought to stop the vote, which they regard as illegal, by seizing voting materials.
Police searches in the Catalan capital, Barcelona, sparked a mass street protest on Wednesday.
Separatist parties who control the Catalan parliament pushed through the referendum law earlier this month after unsuccessfully demanding for years the right to hold a free vote on self-determination.
So far the crisis has been peaceful but police scuffled with Catalan demonstrators this week.
Speaking from his official residence, the Moncloa Palace, Mr Rajoy said there was “still time to avoid greater evils” and he urged Catalan leaders to observe the law.
The referendum, he said, was a “chimera”.
Earlier on Wednesday, Catalan President Carles Puigdemont accused the central government of effectively suspending devolution after it moved to seize control of regional finances to stop them being used to fund the referendum.
“Spain has de facto suspended the self-government of Catalonia and has applied a de facto state of emergency,” he said.
“We believe that the Spanish government has crossed the red line that separated it from repressive authoritarian regimes and has become a democratic shame.”
Protests in Barcelona and other Catalan towns, such as Tarragona and Berga, continued into the night.
In Barcelona, a large crowd of demonstrators remained outside the Catalan economy department, focus of unrest earlier when police detained the regional Economy Minister, Josep Maria Jové. Two other officials were also detained.
Another focus of the protests was the headquarters of the radical CUP party, which backs the referendum.
Referendum supporters have continued to plaster walls and pillars with home-printed posters that read “We vote to be free” and referendum stickers with the single word Si (“Yes”).
Catalonia, a wealthy region of 7.5 million people in north-eastern Spain, has its own language and culture but is not recognised as a separate nation by the Spanish state.