Pro-independence lawmakers hope the northeastern region will gain complete political and economic autonomy from Spain despite the referendum putting Catalonia in open defiance of central authorities in Madrid.
This video shows the unity rally outside Plaza Mayor in Valladolid, in the autonomous community of Castile and León. The government has ordered police and the Guardia Civil, or Spanish Civil Guard, to confiscate all election-related materials, including the actual ballot boxes.
Credit rating agency S&P said that while it did not it expect Catalonia, a wealthy region that borders France, to secede from Spain, protracted tensions between Madrid and Barcelona could have a negative impact on the country's economic growth outlook.
Catalan Quim Roy, a father of two daughters, is occupying a school with other protesters who have organised picnics, yoga sessions and other activities.
Catalonia's defiant regional government is pressing ahead anyway, urging the region's 5.3 million voters to make their voices heard.
"I'm asking the Catalan president to call off the referendum, because if it happens he likely end in a situation in which he will have to appear before a judge", Millo said. The block has only warned Catalans that if they were to secede from Spain they would have to apply for European Union membership, which Madrid would have authority to block.
Police have sealed over half of Catalan schools, designated as polling stations, for the now-banned independence referendum.More news: Australia should prepare for North Korean attack, former Pentagon missile boss warns
Thousands of extra police have been drafted in from across Spain to try to stop it going ahead.
Spanish government spokesman Inigo Mendez de Vigo said Friday that there's no way the regional administration will pull off its plan to hold an illegal referendum on independence on Sunday. In a sign that large crowds are expected on the streets on Sunday, department store chain El Corte Ingles said it would shut three stores in central Barcelona.
Where Do We Go Now? Some shouted "Long live Spain!" and "Puigdemont to jail!"
Catalan separatist leaders and organisers of "committees to protect the referendum" have stressed that everyone must remain peaceful.
Catalan leaders have previously said if the referendum passes, they will declare independence in less than 48 hours.
On Saturday, Guardia Civil officers raided the Catalan government's telecommunications and information technology center, Joan Maria Piqué, the global communications director for the government of Catalonia, told CNN.
Numerous British commentators have suggested that the Spanish government should follow the example of Cameron's coalition government, which granted the Scottish people a referendum. But Madrid snubs Catalan officials by refusing to grant greater financial independence to the region.