The abdication by King Juan Carlos of Spain has sparked calls for a referendum on the country's monarchy as tens of thousands of Spaniards took to the streets across the cities demanding voting.

Madrid, Barcelona and several other cities witnessed demonstrations after the 76-year-old king announced he would be stepping down paving the way for his son Crown Prince Felipe.

Authorities estimate at least 20,000 people stormed the landmark Puerta del Sol square in the Spanish capital while thousands of others descended on Catalunya square in Barcelona calling for the referendum.

One Barcelona resident and protestor said: "We're here, taking advantage of the king's abdication, to ask for a referendum about whether we really want to continue with the monarchy or we want a republic. So all we're asking for is a bit of democracy by the government and they offer us the opportunity to decide."

Several online petitions have also been filed urging the Spanish government to use the opportunity to hold the referendum.

Juan Carlos, who was once a popular monarch as he oversaw the transition from dictatorship to democracy, decided to abdicate after 39 years on the throne. He lost much of his popularity in recent years over various scandals involving him and his family members.

The Spanish cabinet is set to meet to discuss further steps as the country presently does not have a law on abdication and succession.