For more than three decades, King Juan Carlos of Spain has enjoyed the unquestioning loyalty of his subjects and the discreet respect of the media. But the era of deference during which the royal family’s jet-set lifestyle and personal affairs were free of public scrutiny could be drawing to a close.

A recent series of small protests against the king by Catalan nationalists and calls from a conservative commentator for his abdication have broken a longstanding taboo about openly criticizing the royal family and prompted debate about the role of the monarchy and the privileges and uncommon reverence enjoyed by King Juan Carlos.

The king took the unusual step last week of defending the constitutional monarchy, saying it had helped guarantee “the longest period of stability and prosperity that Spain has ever experienced under democratic rule.”

The controversy mirrors a broader debate about the relevance of monarchy in Europe, as members of the younger generation marry nonroyals with checkered pasts and taxpayers question the expense of maintaining their monarchs’ lifestyles. But the challenge in Spain also is rooted in a fervent nationalist mood in regions that resent the centralized power of Madrid, and questions about whether an off-limits monarchy is anachronistic in a modern democracy.

The king’s role in helping restore democracy in Spain after the dictatorship of Gen. Francisco Franco won him the enduring gratitude and respect of a majority of Spaniards.

Born in Rome in 1938, the king returned to Spain at age 10, where he studied and was groomed as Franco’s successor. He was proclaimed king in 1975, after Franco’s death, but steered Spain toward a parliamentary system and now has little power.

During an attempted military coup in February 1981, the king went on television to denounce the putsch and urged the Spanish people to support the democratically elected government.

“He played an incredibly courageous role in the transition,” said Paul Preston, a professor of history at the London School of Economics and the author of several books on Spain. “He won the right to be king.”

The royal family remains popular among Spaniards, who in May voted King Juan Carlos the greatest Spaniard of all time. But the affection for the king himself does not extend to the institution of the monarchy, and many refer to themselves as juancarlistas rather than monarchists.

For decades, the news media have stepped gingerly around the royal family, eschewing gossip and limiting themselves to tame pictures of the king and queen during their many official journeys and of their infant grandchildren in lacy white outfits.

While Spaniards gobble up countless magazines filled with compromising pictures of European royalty and celebrities, magazine editors say they have no interest in seeing vulgar images of their own royal family.

Then in June, the satirical magazine El Jueves, a Catalan publication, published a cartoon that depicted Juan Carlos’s son, Felipe, and his wife having sex, a drawing the editor said was meant as a joke. A judge’s decision to recall the magazine and try the cartoonists on charges of damaging the prestige of the crown set off a frenzy of interest in the little-read publication and furious debate about freedom of speech. Those convicted of damaging the monarchy’s image face up to two years in prison.

“The way we handle our monarchy is very antiquated, and repression has a counterproductive effect,” said Fernando Bouza, a public opinion expert at the Complutense University of Madrid.

The cartoon controversy appeared to inspire others to vent their criticism and fueled resentment among Catalan nationalists and anarchists, political commentators said. It was followed by a series of small rallies by leftist protesters who support the independence of Catalonia, in the northeast, and see the king as a symbol of an oppressive, centralized Spain. In recent weeks, demonstrators have burned placards bearing the king’s image and shouted slogans that included, “The Catalans have no king.”

In a mock lynching last week, two hooded men hung a mannequin wearing a crown from atop the Autonomous University of Barcelona, local reports said.

On Tuesday, a group of protesters in Valencia, a Catalan-speaking region of eastern Spain, burned posters bearing the king’s photo and scuffled with right-wing monarchists, according to the newspaper El País.

Esquerra Republicana de Catalunya, a leftist Catalan party, has lobbied to force the royal household to disclose details of its finances, and in July the royal household appointed an internal auditor to oversee its accounts.

The royal household declined to comment on the recent incidents or the king’s response, as is customary practice.

Miguel Ángel Moratinos, the foreign minister, dismissed the protests as the “antics of a radical fringe,” saying that the king was “a strong figure valued by the vast majority of Spaniards.”

But the attacks have not come solely from the left. Last month, Federico Jiménez Losantos, a conservative radio commentator, called on Juan Carlos to abdicate in favor of his son to allow the regeneration of the monarchy.

Commentators from both sides of the political spectrum said the king also might have become a victim of the very role for which he is so widely revered: aiding the transition to democracy, which was something of a forgive-and-forget process intended to close the door on Spain’s civil war, from 1936 to 1939, and the decades of dictatorship. No war crimes trials were held, and several of Franco’s allies remained in politics and the military.

Despite the recent challenges, many politicians, historians and commentators said they believed that the king would recover.

“This is a country that is basically inclined toward republicanism, but people don’t see a change of system as something that’s needed” Mr. Bouza said.