Tens of thousands of Spaniards streamed into town squares in more than 60 cities across the country, just hours after the king's surprise announcement on national television. Demonstrators chanted, "No to monarchy! Yes to democracy!" and demanded an immediate referendum on whether Spain should remain a constitutional monarchy or become a republic after Juan Carlos, who has been king for 39 years, steps down.
"It is unthinkable that in the 21st century we are still talking about blood rights," said Cayo Lara, the leader of Spain's United Left coalition. "We are not subjects, we are citizens."
Though the king's role is largely ceremonial, many Spaniards credit Juan Carlos with shepherding the country from the military dictatorship of Francisco Franco to democracy in the 1970s.
But the 76-year-old monarch has been ailing in health as well as popularity. Juan Carlos drew public outrage two years ago when he went elephant hunting in Africa while his country was mired in recession. His daughter Infanta Cristina is being investigated on suspicion of tax fraud and corruption. The royal lifestyle has not sat well, especially with Spanish unemployment at 25%.
"The long, deep economic recession we are enduring has left serious scars in the social fabric," Juan Carlos acknowledged in a recorded five-minute video address Monday. "A younger generation with new energy has the determination to transform the country.... My son, Felipe, inheritor of the crown, is the embodiment of stability."
"I want the best for Spain, to which I have dedicated my whole life," the king said.
No Spanish monarch has handed power to his or her offspring since the 19th century. Courts would need to approve Crown Prince Felipe, 46, as his father's successor. Prime Minister Mariano Rajoy, a pro-monarchy conservative, said he planned to hold an emergency Cabinet meeting Tuesday to begin enacting legal and constitutional mechanisms to allow for the change.
Juan Carlos, who was born in Italy in 1938 and raised in Portugal, was plucked from exile and personally groomed by Franco to be his fascist successor. But when Franco died in 1975, Juan Carlos ushered in democracy instead. Then, in 1981, he endeared himself to Spaniards by putting down an attempted coup by paramilitary police who opened fire inside parliament. He is credited with keeping the nation's then-fledgling democracy alive.
"Franco gave the king extraordinary powers, before there was even a constitution, but Juan Carlos used that power to create a democracy instead," said Bieito Rubido, editor of Spain's monarchist ABC newspaper. "And then he intervened to stop that coup, insisting again on democracy. So Spaniards credit him with the peace and progress we've seen since then."
Juan Carlos is the only king Spaniards today have known, and the tanned yachtsman and international jet-setter has been beloved for most of his reign. His family vacationed with Britain's Prince Charles and Princess Diana in the 1980s. When Spain won the soccer European Championships in 2008, one of the team's captains flung his arms around Juan Carlos in a bear hug, in what has become an iconic image of the king as a patron of Spanish sports.
One of his most infamous exchanges came in 2007, at an Ibero-American Summit held in Santiago, Chile. Venezuelan President Hugo Chavez interrupted Spanish Prime Minister Jose Luis Rodriguez Zapatero during a panel discussion, and Juan Carlos snapped at Chavez, "Why don't you just shut up?!" Video of the moment went viral in Spain, and some Spaniards still use the audio as a cellphone ring tone.
A renunciation of the throne had been bandied about as a possibility for some time in local media because of the king's declining health and some of the recent scandals besetting his family. Juan Carlos has undergone several surgeries and broke his hip in 2012.
The king said he made the decision to abdicate after his 76th birthday in January and gave Rajoy his formal notice Monday morning.
In recent months, polls have indicated that a majority of Spaniards said they thought the king should abdicate and that they had a more positive view of Felipe, who has taken on an increasingly larger public role in recent months.
"We want to vote for the next step. I perfectly understand those who support the monarchy, and also those who don't," said Elena Idoate, a 20-year-old student who joined a large rally in Madrid's central Puerta del Sol square late Monday. "But honestly, if our king taught us anything, it's that we're a democracy — and in a democratic system we must vote for the future."