Thousands of protesters gathered in Spain's capital Madrid on Saturday to demand the abolition of the monarchy.
Demonstrators marched in the capital waving flags and banners, and chanting: "Spain tomorrow will be republican."
In Spain's northern Basque Country, an estimated 100,000 protesters formed a 123km (75 miles) human chain linking the Basque town of Durango to Pamplona, the capital of the neighbouring Navarre region.
Waving Basque flags, the protesters called for a referendum to abolish the monarchy and also demanded the right for a vote on the region's independence.
The latest demonstrations come after thousands took to the streets earlier this week when Spain's 76-year-old monarch King Juan Carlos announced his abdication. He is expected to be replaced by his son Crown Prince Felipe later this month.
The protests, with scores of anti-monarchy activists waving republican flags, were among the biggest mass rallies seen in Madrid. Thousands also gathered in Barcelona, chanting: "Dear Felipe, nobody has chosen you."
Juan Carlos was long popular for his role in guiding Spain's transition to democracy in the 1970s, but a series of royal scandals has led to the Spanish royal family falling out of favour with the general public.
In 2012, Juan Carlos provoked public uproar when he went of a secret luxury elephant-hunting trip to Botswana at a time of soaring unemployment.
Earlier this year, Spain's Princess Cristina was questioned in court in connection with a corruption scandal involving her husband Inaki Urdangarin's business dealings.
Urdangarin is alleged to have defrauded regional governments out of millions of euros of public money.
It marked the first time that a member of Spain's royal family has appeared in court in connection with a criminal investigation.
A poll published by Spanish newspaper El Mundo in January found that less than half of Spaniards (41%) supported the monarchy, with almost two-thirds saying they wanted Juan Carlos to abdicate.