"Catalonia belongs to us all, and not just to the nationalists", said Álex Ramos, of the anti-separatist Catalan Civil Society, which organised the protest.
Protesters on Sunday called for the imprisonment of Mr Puigdemont after he pledged to push for independence.
Ramon Tremosa, an European Union lawmaker from PDeCAT, told Onda Vasca radio on Monday that Puigdemont could follow the example of Slovenia, which declared independence in 1991 following an unauthorized referendum, but waited months before enacting it.
"If there were to be a declaration of independence, it would be unilateral and it would not be recognized", French European Affairs Minister Nathalie Loiseau told French broadcaster CNews on Monday.
The songs include "We are all Catalans", "Only one nation" and "Puigdemont in prison".
Mario Vargas Llosa, the Peruvian-born author known for his right-wing views, is attending the rally and has denounced Catalan independence, calling it a nationalist movement that will bring down Spain.
Turnout was 43 per cent as Catalans who reject independence largely boycotted the poll. The latest crisis has raised fears of unrest in Catalonia, a northeastern region about the size of Belgium that is home to 7.5 million people and accounts for a fifth of Spain's economy. Media suggested that the process of declaring independence may be launched during this parliamentary session.
Divisions between Catalan leaders and the central government took a particularly brutal form on the day of the vote, when thousands of Spanish police went to Catalonia to try to shut the referendum down and clashed with protesters and voters.
On Tuesday, Catalan parliamentarians are expected to discuss the results of the referendum.
"Equally I hope the Madrid government will engage for the first time in meaningful talks - there is a lot of pressure on Madrid presently, and I think there is an opportunity for the European Union to facilitate a dialogue". However, Catalans have always been calling for more say in spending, higher status for their language and recognition that they are a nation distinct from Spain, BBC writes.
More than 500 people gathered on Sunday at midday, on the Esplanade du Parlement Européen in Brussels, in support of the anti-independence demonstrations in Barcelona.
Three of the four main parties in the Spanish parliament have clearly stated their support for whatever the Spanish government needs to do to maintain law in the face of the declaration, which could mean suspending the region's autonomy. "We will see what happens this week but we have to speak out very loudly so they know what we want".