Could Juan Carlos be the last Spanish monarch? As the 76-year-old handed the reins to son Felipe, tens of thousands in Spain have rallied to demand the Prince never takes the throne.
More than 60 Spanish cities, from Madrid to Bilbao, Barcelona and San Sebastian, saw protests of thousands on Monday evening, despite the shock nature of the abdication announcement on Monday.
Around 20,000 people gathered with just a few hours notice on the streets of Madrid and marched to Puerta del Sol square. Thousands more still gathered wrapped in Catalonian and Basque nationalist flags in Barcelona and in Bilbao.
Spain protests against the monarchy
A petition has garnered 13,000 signatures in less than 24 hours calling on Spain's politicians to take advantage of the historic opportunity for a referendum on Spain's political future, saying now is the time to "promote a public debate that will help regenerate democracy and determine the future of the monarchy".
Cayo Lara, who heads the United Left coalition called it "inconceivable in the 21st century that we're still talking about blood rights."
Pablo Iglesias, the leader of Podemos party which has five seats, said: “We have to give the people the say. If Felipe Bourbon wants to lead Spain, then the people should vote on it.”
The King had earlier addressed his subjects on television, after prime minister Mariano Rajoy made the announcement of the abdication. "Today, when I look back, I can only feel pride and gratitude to all of you," he said. "Pride for the many good things we have achieved together. And gratitude for the support you have given me throughout my reign."
Constitutional changes are needed for Juan Carlos to abdicate, and to allow his first-born daughter to succeed him. With a major change on the horizon, activists see this as a major opportunity to push for further reforms to the constitution, like greater autonomy from Madrid for regions, and for a referendum on the monarchy itself.
Speaking from Madrid, Vanesa Rodríguez, editor-in-chief of HuffPost Spain said that although Felipe was more popular than his father, there was "large public disaffection" with the monarchy, said Rodríguez, with a survey showing people gave it 3.96 out of 10.
"Most political parties on the left are in favour of opening a process of change now and they want to be able to ask citizens whether to continue with the monarchy or opt for a change," she added.
Juan Carlos has ruled for more than 38 years, but has seen his once immense popularity and that of the Royal Family dwindle in recent years following a number of embarrassments.
He has endured a long-running corruption probe into his younger daughter, the Infanta Christina, and son-in-law Inaki Urdangarin, suspected of fraudulently obtaining millions in public funds.
Once seen as the king with the common touch, that reputation disintegrated after he caused widespread furore in Spain by going on an lavish secret elephant-hunting trip in 2012, at the height of the Spanish financial crisis.