In response to the recent scandals, the Spanish royal household had already become more transparent about its spending. In February, it unveiled an annual budget of 7.8 million euros (about $10.6 million), down 2 percent from 2013. It included a salary of €292,752 (nearly $400,000) for Juan Carlos and half that amount for his son. The household also revealed that the king’s health problems and surgeries had cost just over €165,000 in a year.

But the Spanish royal family has not disclosed the value of its assets, nor provided the kind of details available in Britain, where royal spending is made public down to laundry costs. In the most recent financial year, the British government spent 33.3 million British pounds — or about $60 million — on the monarchy.

Herman Matthijs, a professor of public finance at the University of Brussels who analyzes government spending on Europe’s royalty, said that the Spanish royal family’s annual budget looked “quite cheap” compared to other monarchies, but “on the other hand we don’t know anything about the wealth of the royal family, which is treated like a state secret, and they also really live for free because all the expenditure of their houses and palaces are in the state budget.”

Mr. Matthijs suggested an abdication was “an ideal moment to make some changes and the protests in Spain showed that it might be time to move to more of a protocol system, like in the Scandinavian countries and the Netherlands, where there is less power for the king.”

Image King Juan Carlos of Spain, left, and his son Felipe attended a military ceremony on Tuesday near Madrid. Credit... Daniel Ochoa De Olza/Associated Press

Since Monday, the anti-royal movement has spread beyond the streets of Spain. Helena Fernández de Bobadilla, a Spaniard who works for an insurance broker in London, said she felt that the monarchy was “obsolete” and signed an online petition calling for a royal referendum.

“Spaniards as citizens should have the right to vote which way we want to go forward and the monarchy should earn democratically their right to reign,” she said. “Unfortunately the Spanish royal family has disappointed us a lot lately.”