Thousands of people were rallying on Sunday in central Barcelona to protest against the Catalan government's push for secession from the rest of Spain.
Many in the crowd carried Spanish and Catalan flags .
Some chanted "Don't be fooled, Catalonia is Spain" and called for Catalan president Carles Puigdemont to go to prison.
The rally comes a week after Mr Puigdemont and other separatist leaders of the Catalan government held a referendum on secession that Spain's top court had suspended and the Spanish government said was illegal.
The Yes side won the referendum with 90% of the vote, though fewer than half of the region's electorate voted.
Mr Puigdemont has pledged to push ahead for independence anyway and is set to address the regional parliament on Tuesday "to report on the current political situation".
Spanish Prime Minister Mariano Rajoy has vowed that his government will not allow Catalonia to break away from the rest of the country.
In an interview with the Spanish newspaper El Pais on Sunday, Mr Rajoy said that he will consider employing any measure "allowed by the law" to stop the region's separatists.
Mr Rajoy said that includes the application of Article 155 of the Spanish Constitution, which would allow the central government to take control of the governance of a region "if the regional government does not comply with the obligations of the Constitution".
"The ideal situation would be that I don't have to find drastic solutions, but for that to happen there will have to be some rectifications (by Catalan leaders)," Mr Rajoy said.
Large rallies were held on Saturday in Madrid, Barcelona and other cities to demand that Mr Rajoy and Mr Puigdemont negotiate to find a solution to Spain's worst political crisis in nearly four decades.
Pro-union forces are hoping to gather more steam with Sunday's protest in Barcelona after a series of large businesses, including Catalonia's top two banks, announced they were relocating their headquarters to other parts of Spain.
Other companies are reportedly considering leaving Catalonia to avoid being cast out of the European Union and its common market in the case of secession.
"I hope that nothing will happen. Because (Catalonia) is going to lose more than (Spain) because businesses are fleeing from here already," said protester Juliana Prats, a Barcelona resident.
"I hope it will remain like it has been up until now, 40 years of peace."
Nobel Literature Prize laureate Mario Vargas Llosa and former president of the European Parliament Josep Borrell are expected to address Sunday's rally, organised by the pro-union grassroots group Societat Civil Catalana.
The most recent polls taken before the referendum showed that Catalonia's 7.5 million residents were roughly split over secession.
Organisers said 930,000 people had joined the march. Barcelona police said there were 350,000 protesters.