Spain's King Felipe VI has voiced his opposition to Catalan leaders' bid for secession, urging the state authorities to defend what he calls "constitutional order".

Felipe abandoned his previously measured tone on Tuesday, saying that "it is the responsibility of the legitimate state powers to ensure constitutional order".

He issued the warning in a televised address two days after Catalans held a banned referendum marred by police violence.

He spoke after hundreds of thousands of Catalans rallied in anger at violence by police against voters during a banned referendum on independence for their region on Sunday.

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Catalan regional leaders held the vote in defiance of the national government which labels it illegal.

"With their irresponsible conduct they could put at risk the economic and social stability of Catalonia and all of Spain," Felipe said of the Catalan leadership.

"They have placed themselves totally outside the law and democracy.

"It is the responsibility of the legitimate state powers to ensure constitutional order."

Plea for harmony

Felipe repeated his earlier calls for harmony between Spaniards, but it was a delicate balancing act for the Spanish sovereign after Sunday's violence.

Police unions and political experts say Spain's government risks losing control of the northeastern region.

It is considered Spain's worst political crisis since an attempted military coup in 1981, which was defused by Felipe's father, King Juan Carlos I.

Crowds in Barcelona on Tuesday screamed for national security forces to get out of the region, branding them "occupation forces" and raising their middle fingers at a police helicopter circling overhead.

City police put the number of demonstrators at 700,000.

"The government of Catalonia is pushing the population to the abyss and inciting rebellion in the streets," Juan Ignacio Zoido, Spain's interior minister, said on Tuesday.

He said his government would take "all measures necessary to stop acts of harassment".

Pictures of police beating unarmed Catalan voters with batons and dragging some by the hair during Sunday's ballots drew international criticism.

Carles Puigdemont, Catalan's regional leader, 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.

Claims for statehood

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 statehood date back centuries but have surged during recent years of economic crisis.

The Spanish central government blames the Catalan regional authorities for the tensions.

Puigdemont declared on Sunday that Catalonia had "won the right to an independent state".

His regional government claimed that 2.26 million people took part in the poll, or just over 42 percent of the electorate, but the vote was held without regular electoral lists or observers.

The regional government said 90 percent of those who voted backed secession, but opinion polls indicate Catalans are split.

Puigdemont has said he will present the results to the region's parliament, where separatist legislators hold a majority.