Thousands of people demonstrated in Barcelona on Tuesday to join a rally demanding independence for Catalonia, in north-eastern Spain, on the Catalan national day.

It has been three centuries since Catalonia was independent from Spain, but the day when it regains autonomy seems closer.

Artur Mas, president of Catalonia's Parliament, announced a referendum on the region's independence in November 2014 – which the central Spanish government immediately damned as unconstitutional. Spain's constitution speaks of the "indissoluble unity of the Spanish nation."

Together with Scotland's push for independence, the Catalonian campaign marks a mounting wish by historically autonomous states, which have become provinces, to assert their independence as the euro zone crisis makes their parent states look weaker.

(Read more: Scottish independence: Widnae it work?)

Both places have in common fierce pride in their cultural heritage, a population size which would place them among the European Union's smaller countries, semi-autonomy at the moment, and awareness of their economic importance – Catalonia as a source of exports and Scotland as the part of the U.K. closest to its North Sea oil fields. Agitators for the independence of both regions want to stay within the EU and keep their current currencies.

"Given the economic importance of Catalonia , the degree of uncertainty around the issue could unsettle markets, initially Spanish sovereign bond markets, if not in the run-up to a ballot then in the aftermath of a possible double "yes" vote," Alastair Newton, senior political analyst at Nomura, told CNBC.

(Read more: Why Catalonia should worry markets)

Catalonia accounts for roughly one-fifth of Spain's gross domestic product (GDP), with around 15 percent of its population, and its GDP per capita is the highest in the country. However, Catalonia has been bailed out by the central Spanish liquidity fund to the tune of 14.5 billion euros ($19.9 billion), and it is unclear how the repayment would work if Catalonia is autonomous.