Influential separatist citizens' organisations urged those who want the vote to take place on October 1st, despite Madrid's ban, to return to the streets after day-long demonstrations that lasted well into the night.
Anger over the detention of 14 regional government officials on Wednesday was further compounded by an announcement that police had seized "close to 10 million ballot papers" destined for a vote deemed illegal by Madrid and the courts.
Thousands took to the streets of Barcelona, prompting Spanish Prime Minister Mariano Rajoy to call on Catalonia's separatist leaders to "stop this escalation of radicalism and disobedience once and for all".
Spontaneous chants in BCN rally, still running at 11pm: "Streets will always be ours" pic.twitter.com/ZlmGaH9uVf— Catalan News (@catalannews) September 20, 2017
"There is still time to avoid bigger problems," he said in a televised statement on Wednesday evening as protesters gathered in front of the regional vice-presidency in Barcelona, threatening to "spend the night here".
They blocked the departure of officers from the Guardia Civil police force who had been conducting searches in the building, and in the early hours of Thursday morning, several hundred remained.
Threats and seizures
The crisis in Catalonia has deepened almost daily over the past weeks as central authorities pull out the stops to prevent the referendum in a region sharply divided over independence.
Police have seized over 45,000 notifications destined for Catalans selected to staff polling stations, threatened to arrest mayors who facilitate the vote if they do not comply with a criminal probe and tightened control over the region's finances.
This and other measures are making it harder and harder for Catalonia's executive to stage the referendum, and Spain's leading El Pais and El Mundo dailies said Madrid had dealt a major blow to the organisation of the vote.
Among those arrested on Wednesday was Josep Maria Jove, secretary general of economic affairs and Catalonia's deputy vice president, a regional government spokesman said.
The others work in various Catalan government departments and are suspects in a probe into "disobedience, misfeasance and embezzlement", the High Court in Catalonia said.
Launched in February, the probe centres around allegations that confidential data was stolen to provide separatists with information on Catalan taxpayers, a judicial source who refused to be named said.
Catalan President Carles Puigdemont condemned the "totalitarian and undemocratic attitude of the Spanish state".
He accused Madrid of imposing a "de facto" state of emergency to stop the referendum.
'Full of protests'
Throughout Wednesday, protesters rallied in front of several regional government buildings and the headquarters of the radical CUP party, part of the ruling separatist coalition in Catalonia, where police said they were conducting an unspecified "operation".
Supporters of the referendum in Catalonia also protested in several cities in the region and in Madrid.
"We're here more by pride than for independence," Marc Oltra i Garcia, a 23-year-old gardener, said in Barcelona.
"If we don't get independence today it will happen tomorrow, and if not tomorrow, after tomorrow."
Omnium and the Catalan National Assembly, two influential separatist organisations, called for more rallies in front of the high court in Barcelona on Thursday -- protests they pledged would be "permanent" until those detained are freed.
Polls show that while Catalans are sharply divided on whether they want independence or not, a large majority would like to vote to settle the matter.
But Madrid is against it, pointing to the constitution which states that the unity of the Spanish nation is "unbreakable" and that only the central government has the power to call a referendum on any matter.
Separatists in Catalonia, a region with its own language and customs, have responded they have a democratic right to decide on their future.
Pro-separatist parties captured 47.6 percent of the vote in a September 2015 Catalan election which was billed as a proxy vote on independence, giving them a narrow majority of 72 seats in the 135-seat Catalan parliament.
But a survey commissioned by the regional government in July showed that 49.4 percent of Catalans were against independence while 41.1 percent were in favour.