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.
Casting doubt on the validity of the vote, she described Spain as a "great democracy" and pointed to the "particularly" high level of devolution its regions already enjoyed.
Williams said the letter would be posted later Monday on the Nobel Women's Initiative website.
The country's courts judged the vote to be illegal and unconstitutional but the regional Catalan government made a decision to press ahead with it.
- Catalan president Carles Puigdemont expected to present referendum results before parliament on Tuesday. He says he will address the regional parliament on Tuesday " to report on the current political situation".
Pro-independence leader Carles Puigdemont is coming under increasing pressure after a number of Catalan-based companies re-located outside of the region in the wake of an unofficial independence vote just over a week ago.
Mr Rajoy told Catalan leaders that there is still time to backtrack and avoid Madrid taking over the region's government.
"We reach out for dialogue but we'll support the response of the rule of law in the face of any attempt to break social harmony".
"If there were a declaration of independence it would be unilateral and it wouldn't be recognised", Nathalie Loiseau said on CNews digital news channel.
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However, Mr Puigdemont's plans were derailed by the Constitutional Court which ruled that a special plenary session to debate the referendum results would be suspended on Monday.
Puigdemont, a former journalist and not a career politician, said he is not afraid of going to jail over independence.
The 1 October independence referendum was, however, marred by riots and clashes between police and voters and declared illegal by Spanish Prime Minister Mariano Rajoy.
Around 350,000 people attended, municipal police said, while organisers put turnout at between 930,000 and 950,000.
Despite the outrage, Rajoy and Spanish King Felipe VI defended the police and doubled down on their stance to take whatever measures necessary to keep Catalonia from seceding.
In an interview with El Pais newspaper, Rajoy also rejected any mediation to resolve the crisis, BBC reported on Sunday.
However, SNP MEP Alyn Smith urged caution and said a declaration of independence from Catalonia would not help its cause at the moment, and that the Catalan and Spanish governments urgently need to open up a dialogue. "We'll do everything that legislation allows to ensure that".
With its own language and cultural traditions, demands for independence in Catalonia date back centuries but have surged during recent years of economic hardship.
Such a move "will not go unanswered by the government", Deputy Prime Minister Soraya Saenz de Santamaria warned on Monday, speaking on radio station COPE. Placards also bore the message "We are Catalan".
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