Longtime former Catalan leader Jordi Pujol was under observation at a Barcelona hospital on Tuesday a day after suffering a stroke.
The 92-year-old, who served as leader of the wealthy northeastern Spanish region from 1980 to 2003, underwent a successful operation but it was "too early to assess the potential neurological consequences", Alberto Lleo, head of neurology at Sant Pau hospital, told reporters.
The next few hours could be crucial, he said.
Known as the architect of modern-day Catalonia, Pujol managed to gain more autonomy for the region during his 23 years in power thanks to his skill at negotiating with the government in Madrid.
An ardent champion of Catalan nationalism, for which he spent time behind bars during Francisco Franco's dictatorship, Pujol became a figurehead for the movement during his time in office when he led the centre-right Convergence and Union party.
A father of seven and polyglot who speaks six foreign languages, he in 2013 threw his weight behind the burgeoning separatist movement that culminated in the 2017 failed independence bid.
At the time, Pujol was embroiled in an illegal party funding scandal, being formally named as a suspect on charges of fraud and money laundering in December 2014.
Six months earlier, he had stunned supporters by admitting that his family had hidden an inheritance from his father in an offshore account in Andorra, a small principality in the Pyrenees between Spain and France.
But he denied accusations that the money came from corrupt activities during his tenure as Catalan leader.