Spanish newspapers gave a mixed response Saturday to King Juan Carlos's first public interview in more than a decade, which aimed to polish his image after a year blighted by scandals.

Major right-leaning newspapers wished him a happy 75th birthday -- the occasion for the interview broadcast on Friday evening -- with the exception of major title El Mundo.

It pointed out in an editorial that the interview on television channel TVE avoided questions on the scandals that struck him in 2012.

Conservative daily ABC praised him in an editorial under the headline "Spain needs the monarchy."

Leading left-leaning daily El Pais lead on his tacit swipe at Catalonia's drive for independence and his call for unity in the economic crisis.

It remained to be seen how the speech had affected the King's popularity in opinion polls.

A poll published Thursday by El Mundo newspaper showed only half of people expressed a positive judgement of his reign, compared with three-quarters a year ago.

Juan Carlos won wide respect for helping guide Spain to democracy after the death of dictator Francisco Franco in 1975, and helping to quell an attempted military coup in 1981.

But last year he was hit by two scandals that raised questions over the monarchy's future.

The first was a corruption case against his son-in-law Inaki Urdangarin, the duke of Palma, who went before a judge in February.

The second was an expensive game-hunting trip the king himself made to Botswana, seen as an unacceptable extravagance while Spain suffered in a recession.

"Not asking about Urdangarin nor the Botswana episode was a mistake," El Mundo said of the interview, in an editorial.

"The questions that were not asked could end up becoming news."

Observers saw the interview as one in a series of efforts to strengthen the monarchy, which have included publishing its accounts and overhauling its website.

In Catalonia, the left-leaning El Periodico acknowledged the King's historical role and emphasised the need for "greater transparency".

The more conservative Catalonia daily La Vanguardia hailed the king as "a great example of positivity".