Spain on Thursday marked 40 years since the adoption of its current constitution – a landmark document that saw the transition from a reactionary and brutal military dictatorship to parliamentary democracy – amid a strong political polarization and disunity brought about by separatist tensions in the distinct regions of Catalonia and the Basque Country.

A solemn ceremony held in Madrid saw the attendance of members of both houses of parliament, the sitting government, the Spanish royal family and the country's former prime ministers, as well as the surviving "Fathers of the Constitution."

"The 1978 constitution, the base upon which democracy was consolidated, was the biggest political success in contemporary Spain," said King Felipe VI in a speech, adding that it was still necessary to continue building on that foundation.

The monarch endorsed the notion of adapting the text to what he described as a country "open to change" with a "critical but constructive spirit."

In a context of a strong pro-independence movement in Catalonia, which led to a constitutional crisis that has consumed Spanish politics for the past year, Felipe underscored Spain's territorial, cultural and linguistic plurality as recognized in the document being celebrated.

Prime Minister Pedro Sanchez marked the anniversary on Twitter by calling on the Spanish people to "recover the spirit of concord" present in 1978 – three years after the death of right-wing dictator Francisco Franco, who ruled the country with an iron fist for four decades and relentlessly persecuted any hint of democratic opposition.

"Today, reforming (the constitution) is reinforcing it," he added.

While conservative politicians such as Popular Party leader Pablo Casado have argued that the constitution is in no need of reform and announced that they would block any attempts to change it, the center-left has long called for what it says is an urgently-needed update to the text.

Pablo Iglesias, the leader of the progressive Podemos party, has defended amending the constitution in order to advance toward a republican form of government, as he believed the head of State "should be chosen through an election, not through fecundation."

The far-right party VOX has said it would only support changing the document if it meant "more Spain and less autonomous regions," in reference to the fairly decentralized territorial distribution of powers introduced by the constitution that gives regions significant latitude when it comes to decisions on matters such as education or healthcare, for example.

The constitution, whose language was drafted by members from across the political spectrum (including the recently-legalized Communist Party), was ratified in a referendum by 91.81 percent of eligible Spanish voters on that chilly Wednesday in 1978.