"We need... To start the process to set up an independent state in Catalonia," he said in a speech to the wealthy region's parliament, which was followed by loud applause.
After months of in-fighting, Catalonia's pro-independence faction that won regional parliamentary elections in September finally came to an agreement this weekend over who should lead the new local government.
With Mas stubbornly refusing to step aside as a weekend deadline to form a government loomed, Catalonia seemed to be heading for fresh elections, which would have been the fourth since 2010.
But at the last minute, Mas agreed to step aside on Saturday, naming the relatively unknown journalist and politician Puigdemont as his successor.
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This is expected to open the door for the now-united separatist lawmakers, who form an absolute majority in Catalonia's parliament, to vote in Puigdemont as new president later on Sunday evening.
He told parliament that apart from launching the secession process, Catalonia would need to offer to negotiate "with the Spanish state, the European Union and the international community."
The last-ditch agreement to form a Catalan government stands in stark contrast to the situation in Madrid, where the national government is in limbo following inconclusive December polls.
Incumbent Prime Minister Mariano Rajoy's conservative Popular Party (PP) came top in the December 20 elections but lost its absolute majority, leaving him struggling to form a coalition government.
"The (separatist) coalition is profiting from the power vacuum in Madrid," headlined online daily El Espanol today.