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.
Turnout figures were impressive, but were still lower than what was achieved by the pro-independence camp in an October 3 general strike, when 700,000 people took to Barcelona's streets, according to Guardia Urbana estimates. "Ideally, we should not have to resort to drastic solutions, but in that case there would have to be rectifications".
Williams said the letter would be posted later Monday on the Nobel Women's Initiative website.
The Catalan authorities say more than 90 percent of those who voted backed secession, but opinion polls on the issue suggest the region is more closely divided.
The country's courts judged the vote to be illegal and unconstitutional but the regional Catalan government chose to press ahead with it.
Three banks as well as gas and water companies have announced plans to move their corporate headquarters out of the region, as Catalonia is plunged into a period of instability. Spain is likely to use military force if Catalan leaders declare independence.
Yet politicians supporting Puigdemont's minority government and civil society groups backing independence say they will not accept anything less than a full declaration of independence.
The Prime Minister also said he planned to keep extra police deployed to Catalonia before the referendum until the crisis was over.
Spain faces a week of deep political uncertainty as the secessionist leader of Catalonia considers whether to make a unilateral declaration of independence, against the backdrop of a bitter standoff with the central government in Madrid.
Many Catalans have long highlighted the region's differences from the rest of Spain but the latest surge for independence began in 2010, when Spain's top court struck down key parts of a charter that would have granted Catalonia greater autonomy and recognized it as a nation within Spain.
Puigdemont is set to address the Catalan parliament at 5pm (UK time) on Tuesday to provide an update on "the current political situation".
Some protesters called for Puigdemont to go to jail for holding the independence vote.
"If independence were to be recognised - which is not something that's being discussed - the most immediate outcome would be that [Catalonia] automatically left the European Union".
Spain and the European Commission, the European Union's executive branch, maintain that Catalonia's independence referendum was illegal. Around 350,000 people attended the rally, municipal police said, while organisers put turnout at between 930,000 and 950,000.
Spanish prime minister Mariano Rajoy told Die Welt that the province would not be allowed to secede. Rajoy has warned Catalonia not to act on an October 1 independence referendum.
"I'm calling on the sensible people in the Catalan government. don't jump off the edge because you'll take the people with you".
"If they declare independence, there will be decisions to restore the law and democracy", said on Monday during a radio interview.