In his first interview since a disputed vote on Sunday, Carles Puigdemont said his government would "act at the end of this week or the beginning of next".
Meanwhile, Spain's King Felipe VI said the vote's organisers had put themselves "outside the law".
He said the situation in Spain was "extremely serious", calling for unity.
Tensions between Spain and the north-eastern Catalonia region continue to mount.
The Spanish High Court said it had begun an investigation into key Catalan figures on Wednesday on suspicion of sedition - inciting rebellion against the state - including the head of Catalonia's regional police.
Josep Lluis Trapero, who leads the Mossos d'Esquadra force, is suspected of failing to control a demonstration during a police raid on Catalan government offices before the vote.
Hundreds of thousands of people across Catalonia have been protesting over Spanish police violence during the vote, during which nearly 900 people were hurt.
The police were trying to enforce a Spanish court order suspending the vote, which the government had declared illegal.
During the vote, 33 police officers were also injured, local medical officials said.
In the BBC interview, Catalonia's President Carles Puigdemont said his government would "act at the end of this week or the beginning of next".
When asked what he would do if the Spanish government were to intervene and take control of Catalonia's government, Mr Puigdemont said it would be "an error which changes everything".
Mr Puigdemont said there was currently no contact between the government in Madrid and his devolved administration.
He disagreed with the European Commission's statement on Monday that events in Catalonia were an internal issue for Spain.
The European parliament is due to debate the situation later on Wednesday.
When the speech ended, customers in this city centre bar thumped tables and whistled contemptuously, then quickly resumed normal conversation - King Felipe may as well have not spoken.
It was the things he omitted that rankled - no words about those shocking scenes of police beating voters on Sunday, no urgent appeal for dialogue between the Spanish and Catalan governments, no acknowledgment of the real hunger here for independence or at least a proper, legal referendum, not even a word or two of Catalan.
Instead, he expressed the position of the government, echoing its firm opposition to the vote, saying Catalan leaders had positioned themselves outside the law. He guaranteed "democratic coexistence" on Spanish terms only.
It was a missed opportunity to push the two sides towards dialogue, one customer told me afterwards.
"It doesn't help the situation at all," said another. "I was not expecting him to intervene at all, actually, but he should at least have mentioned the violence here two days ago."
Huge protest rallies have been taking place across Catalonia as well as a general strike.
In Barcelona, 700,000 people took to the streets on Tuesday, city police were quoted as saying.