Spanish PM Mariano Rajoy has warned he would not rule out anything "within the law" to halt Catalonian independence.
Asked if this would include using Article 155 of the Constitution, the legal mechanism needed to suspend Catalonia's autonomy, Rajoy said: "I am not absolutely ruling out anything that the law allows".
Thousands of people gathered Saturday in Madrid to defend the unity of Spain, the Constitution and the rule of law in the face of an independence challenge and a possibility of an imminent unilateral declaration of independence in Catalonia.
"The government will resist the situation, when any declaration of [Catalonia's] independence would have any results", Rajoy said in an interview with El Pais newspaper published on Saturday.
Some European officials are also anxious that any softening in Spain's stance towards Catalan independence could fuel secessionist feelings among other groups in Europe such as Belgium's Flemings and Italy's Lombards.
"We have to find a new way forward", said Miquel Iceta, the leader of Spain's Socialist party in Catalonia.
"Spain will be Spain and will continue to be it for a long time", the prime minister added.
The Catalan authorities say around 90 percent of those who voted supported a split from Spain. Madrid responded to the vote with force, sending thousands of police to the region to shut down the vote.More news: Suspected gunman may have targeted Lollapalooza, other Las Vegas festival
A former Prime Minister of France has said the idea of Catalonia receiving independence from Spain would be "madness".
About 10 companies have made a decision to relocate their legal headquarters from Catalonia to other places in Spain over fears of an unilateral declaration of independence, media reported on Saturday.
There is speculation that the parliament will declare independence unilaterally at its next sitting, based on the referendum.
Catalonia, a region of around 7.5 million people with its own distinct language and culture, has had a complex relationship at times with Madrid.
In a separate rally in Madrid's Colon Square, thousands clamored for the unity of Spain and against any attempt by the northeastern region to break away.
Raul Briones, 40 wearing a Spanish national soccer team shirt said: "The people who have come to demonstrate don't feel Catalan so much as Spanish".
Carles Puigdemont, Catalonia's president, right, and Oriol Junqueras, leader of Esquerra Republicana de Catalunya, join a rally calling for independence prior to the referendum. "I think a couple of years ago there were more people who didn't want it (independence), but after all that has happened, I think there are more people who want it".
The referendum was declared illegal by Spanish authorities and turnout was only 43 per cent. "I can tell you with absolute frankness that it will not happen", Rajoy said.
The Falange party, which is widely seen as a fascist, right-wing fringe movement in Spain, was founded in 1934 and later led by General Franco, whose right-wing nationalists won the 1936-1939 civil war against left-wing Republican government, leaving some 400,000 people dead.