Spanish lawmakers opened a historic session Wednesday to approve 76-year-old King Juan Carlos's abdication, paving the way for his son Felipe to take the scandal-hit throne despite anti-royalist protests.

Nine days after Juan Carlos called an end to a 39-year reign that guided Spain from dictatorship to democracy, parliament prepared for the future King Felipe VI to inherit the crown.

Prime Minister Mariano Rajoy opened the debate by defending the king and the monarchy, which he called "the best symbol of the unity of the state".

"Spain is a parliamentary monarchy with deep roots because Spaniards want it to be so," he added.

The abdication law is backed by the ruling conservative Popular Party, the main opposition Socialists and the small centrist UPyD party, which together have 300 seats in the 350-seat lower house of parliament.

Once passed by the lower house, the succession will then have to be approved by the Senate, Spain's upper house of parliament, which will vote on the bill on June 17.

The 46-year-old Prince Felipe is expected to be sworn in by parliament on June 19.

Anti-monarchist activists called for protests outside the building during the debate.

The succession must be enshrined in law under Spain's 1978 constitution.

Juan Carlos won widespread respect for defending Spain's democracy, notably appearing on television to thwart an attempted military coup in February 1981.

But gaffes and a corruption scandal centred on his youngest daughter Cristina and her husband Inaki Urdangarin sent his popularity diving in the dying days of his reign.

His son Felipe, a former Olympic yachtsman married to glamorous former television news presenter Letizia with whom he has two daughters, eight-year-old Leonor and seven-year-old Sofia, commands greater popular support.

Nevertheless, tiny left-wing and regional parties, including the United Left coalition and the Catalan separatist Catalan Republic Left, have said they will vote against the law and instead call for a referendum on the future of the monarchy.

A few Socialist lawmakers have asked for a free vote on the law.

During the debate Socialist leader Alfredo Perez Rubalcaba defended Spain's 1978 constitution, which established a parliamentary democracy with the king as a mostly ceremonial head of state saying it "paved the way for peace".

Other parties, such as the conservative Catalan nationalist CiU party, plan to abstain.

Within hours of the king's announcement on June 2 that he was abdicating, thousands of people massed in central Madrid and other cities to demand a referendum on the monarchy, which was only restored in Spain in 1975 after the death of General Francisco Franco.

A majority of Spaniards, 62 percent, want a referendum on the future of the monarchy at some point, according to a Metroscopia poll published Sunday in top-selling centre-left newspaper El Pais.

If such a plebiscite were held, nearly one voter in two, or 49 percent, would prefer to have a monarchy with Felipe as king while 36 percent would support a republic, according to the poll.

A separate survey by Sigma Dos published by centre-right newspaper El Mundo on Monday found popular support for the monarchy has climbed since Juan Carlos announced his abdication.

Overall, almost 56 percent said they wanted Spain to remain a monarchy, up from a historic low of almost 50 percent when the same question was posed in January.

Support for the monarchy is weakest among younger Spaniards, who do not recall the king's role in steering Spain to democracy and who now bear the brunt of Spain's sky-high jobless rate.

Felipe urged Spaniards to unite for a better future during his first speech since his father announced his abdication, in what was seen as a reference to growing independence drives in Catalonia and the Basque Country.