The 49-year-old king abandoned his previously measured tone over tensions with Catalonia as the standoff dragged Spain into its deepest political trouble in decades.
He spoke after hundreds of thousands of Catalans rallied in fury at violence by police against voters during a banned referendum on independence for their region on Sunday.
Catalan regional leaders held the vote in defiance of the national government which brands it illegal - as did Felipe on Tuesday.
"With their irresponsible conduct they could put at risk the economic and social stability of Catalonia and all of Spain," he said of the Catalan leadership.
"They have placed themselves totally outside the law and democracy," he said.
"It is the responsibility of the legitimate state powers to ensure constitutional order."
Police unions and political experts warned that Spain's government risks losing control of the north eastern region as the country plunges deeper into its worst political crisis since the death of the dictator Francisco Franco in 1977.
Crowds in Barcelona yelled for national security forces to get out, branding them "occupation forces" and raising their middle fingers at a police helicopter circling overhead.
Barcelona football club refused to train as part of a strike.
Officials said the strike slowed down public transport and paralysed freight shipments in the port of Barcelona.
It also closed down tourist sites such as the city's emblematic Sagrada Familia Church.
Protesters rallied in the afternoon and a further demo started in the early evening, with demonstrators dancing in a central Barcelona avenue and beating drums in a carnival atmosphere.
"The streets will always be ours," young protesters yelled.
City authorities put the number of demonstrators at 300,000.
Anger at police 'repression'
Pictures of police beating unarmed Catalan voters with batons and dragging some by the hair during Sunday's ballots drew international criticism.
Some demonstrators said they were not pro-independence but came out to protest Tuesday in anger at the police violence.
"The important thing is for the people to be united," said one such voter, Rachael Moya, a social worker.
"I am here because of what happened the other day. The people should be allowed to vote and we should fight against police repression."
Catalan regional leader Carles Puigdemont said nearly 900 people had received medical attention on Sunday, though local authorities confirmed a total of 92 injured. Four were hospitalised, two in serious condition.
The national government said more than 400 police officers were hurt.
The royal palace in Madrid told AFP that King Felipe VI would make an address to the nation on Tuesday evening at 1900 GMT.
The monarch has maintained a cautiously conciliatory stance as tensions in Catalonia have risen over recent years.
Protesters overnight besieged Catalan hotels where state security forces were lodged, police groups said on Tuesday.
"They are fleeing from hotel to hotel, hiding like rats," said the spokesman for Spain's main police union SUP, Ramon Cosio.
Two hotels said local authorities had ordered them to ask police officers staying there to leave.
Spanish authorities "are losing control, it is clear," Javier Perez Royo, a constitutional law professor at the University of Seville, told AFP.
"And the government of the nation is taking steps in a very dangerous direction."
A rich industrial region of 7.5 million people, Catalonia accounts for a fifth of Spain's economy.
It has its own language and cultural traditions.
Its claims for independence date back centuries but have surged during recent years of economic crisis.
Spain's national government and courts have ruled the independence referendum illegal and Madrid blames the Catalan regional authorities for the tensions.
"We see how day after day the government of Catalonia is pushing the population to the abyss and inciting rebellion in the streets," Spain's Interior Minister Juan Ignacio Zoido said on Tuesday.
He said his government would take "all measures necessary to stop acts of harassment".
The government of Prime Minister Mariano Rajoy held emergency talks after Puigdemont declared Sunday that Catalonia had "won the right to an independent state".
Puigdemont has appealed for international mediation to help solve the crisis.
The regional government said 2.26 million people took part in the poll, or just over 42 percent of the electorate.
It said 90 percent of those who voted on Sunday backed independence, but polls indicate Catalans are split.
Puigdemont has said he will present the results to the region's parliament, where separatist lawmakers hold a majority.