The groups were seen hurling metal chairs and tables at one another in Barcelona before police moved in to break up the violence.
Families with Spanish flags were seen fleeing as chairs were used as missiles when the two camps faced off.
Nationalists were heard chanting “Viva Espana” with others performing Nazi salutes as the violence raged.
The shameful scenes came about as a peaceful pro-unity demonstration filed through the city streets.
Carnage broke out on the fringes of the march as tensions mounted over whether Catalonia’s regional government is declaring independence following its unofficial referendum.
In the capital Madrid, Spaniards lined the streets, hanging national flags from balconies as a military parade passed by.
The celebration marks the Christian conquest and Christopher Columbus’s arrival in the Americas in 1492.
Catalan government chief Carles Puigdemont had stopped short of moving a motion of independence in the regional parliament, disappointing many of his own supporters.
But Prime Minister Mariano Rajoy has given the Catalan leader five days to clarify whether or not he has declared independence.
If Mr Puigdemont does not respond in time or confirms a will to secede, Mr Rajoy can turn to article 155 of the 1978 constitution which allows him to sack the regional government.
Ahead of the National Day, Mr Puigdemont tweeted: ”Ask for dialogue and they answer you by putting the 155 on the table. Understood.”
But Mr Rajoy warned: “There is an urgent need to put an end to the situation that Catalonia is going through – to return it to safety, tranquillity and calm and to do that as quickly as possible.”
Uncertainty remains over what article 155, which has never been invoked before, would mean in practice.
Political analysts expect Mr Rajoy’s next step would be to call a snap regional election to plot a path out of the crisis.
King Felipe presided over the Madrid parade, which included almost 4,000 military and police officials.
Last week, in a rare intervention, he accused Catalan secessionist leaders of shattering democratic principles and dividing Catalan society.