Holding aloft a giant banner with the Spanish and Catalan flags, the pro-unity demonstrators flooded Catalonia square in Barcelona, the northeastern region's capital, a month after Catalan separatists formed a massive human chain to call for Catalonia's independence.
"The silent majority has broken its silence. The pro-independence human chain is not the only image of Catalonia," said rally organizer Alicia Sanchez-Camacho, the President of Spain's ruling conservative Popular Party in Catalonia.
City officials said 30,000 people demonstrated, while organizers put the number at 160,000.
The human chain on Catalonia's own national day, September 11th, stretched hundreds of kilometres (miles) and joined hundreds of thousands of people.
Catalonia has vowed to press ahead with plans to hold a self-determination referendum.
But the government of Prime Minister Mariano Rajoy has insisted that a referendum in Catalonia — a wealthy region home to about 7.5 million of Spain's 47 million people — would be unconstitutional.
Spain's national day commemorates the anniversary of Christopher Columbus's first arrival in the Americas on October 12th, 1492.
Crown Prince Felipe presided over the ceremonies for the first time, as Spain's 75-year-old King Juan Carlos was recovering from a hip operation after leaving hospital on October 1st.
Surgeons replaced the king's artificial left hip in the latest of a string of operations that have raised questions over his reign and fuelled speculation of a possible abdication.
Felipe presided over a military parade in Madrid, dressed in military uniform and accompanied by his wife Letizia as they briefly met with troops from the Royal Guard.
The parade lasted barely an hour and was a toned-down affair, in line with crisis-hit Spain's austerity regime.
The budget for the event has been cut by another 15 percent from last year and is now at €823,000 ($1.1 million), down from €2.8 million in 2011.
Juan Carlos is widely respected for his role in guiding Spain's transition to democracy after the death of longtime dictator General Francisco Franco in 1975.
His image has suffered, however, in particular because of a corruption scandal implicating his youngest daughter Cristina, who was not present at the parade.