Nobel Peace laureate Jody Williams told The Associated Press the letter has so far been signed by seven more awardees, including Mairead Maguire, Rigoberta Menchu and Shirin Ebadi.
Thousands of protesters take to the streets during a supporting the unity of Spain on 0810-2017.
The 81-year-old writer and journalist said it would "take more than a coup plot to destroy what has been built in 500 years of history".
Williams said the letter would be posted later Monday on the Nobel Women's Initiative website.
Separatists' roadmap for independence consists of a declaration of independence followed by a series of legislative steps and votes to set up a new state, which could take months.
Saturday saw water management company SGAB announce the relocation of its legal headquarters to Madrid, Friday saw Gas Natural Fenosa announce the same decision adding that this could be a temporary measure due to the political situation of uncertainty Catalonia is going through.
That raises the prospect of Spanish police arresting Puigdemont and other separatist leaders if they declare independence.
Mr Rajoy told Catalan leaders that there is still time to backtrack and avoid Madrid taking over the region's government.
On Tuesday King Felipe addressed the nation in a televised speech but failed to acknowledge the violence perpetrated against Catalan voters, and said nothing of the need for mediation to resolve the crisis.
France will not recognise Catalonia if the Spanish region unilaterally declares independence, its European Affairs Minister, Nathalie Loiseau, said on Monday.
Rajoy has issued a stern warning to Catalan leaders who have said they could declare independence this week. During the event, Vargas Llosa said that though nationalism is wreaking havoc in the region right now, Spanish democracy is here to stay, and no plot for independence can destroy it.
"If independence were to be recognised - which is not something that's being discussed - the most immediate effect would be that [Catalonia] automatically left the European Union".
The apparent threat comes amid heightened tensions between Catalonia and Spain as the wealthy region threatens to break away following an independence referendum held on October 1. That figure was disputed by the local police force which estimated there were still very significant 350,000 demonstrators.