"To all Catalans, to all Spaniards, I want to tell you to maintain confidence in the future as authoritarian delusions (...) will never defeat the serenity and harmony of our democratic state," he said at a gathering in Madrid.
His comments came a day after the separatist coalition that governs Catalonia, a wealthy region of 7.5 million inhabitants with its own language and customs, unveiled a bill aimed at ensuring an independence referendum takes place despite Madrid's refusal.
Lawmakers who form the coalition said that Catalonia would declare independence "immediately" if the region's voters opt to separate from Spain in the vote planned for October 1st.
If the "no" side wins, new regional elections will be called, they added.
The bill is aimed at extracting the region from Spain's legal system in a bid to circumvent all legal and practical challenges to organising a referendum.
It will be submitted to a vote in the Catalan regional parliament, where separatists hold a majority, at the end of August.
For years separatist politicians in the region have tried to win approval from Spain's central government to hold a vote similar to Scotland's 2014 independence referendum from Britain -- which was approved by London, though it resulted in a "no" vote.
But Madrid has remained steadfast in its opposition to such a vote, considering it a threat to Spain's unity.
The Constitutional Court has already quashed a resolution approved by Catalonia's parliament calling for the referendum to take place.
It has also warned Catalonia's elected officials that they would face legal consequences if they took any steps towards holding such a vote.