A delegation of eight MEPs from Belgium, France, Slovenia, Spain, Sweden and the UK praised the high level of participation in the unsanctioned poll "despite the challenges" faced.
Voter turnout out on Sunday was over 30 percent, with over two million people casting their ballot in a symbolic poll which went ahead despite repeated attempts from Spain's central government in Madrid to block the ballot. Over 80 percent of those voters said they were in favour of a split from Spain.
The head of the delegation of observers, the UK's Ian Duncan, noted in a report that the vote "took place in a calm and open manner where no one was coerced or intimidated."
Voting on Sunday was generally peaceful, but a group of people tried to destroy ballot boxes in the town of Girona, with two being arrested. In another incident, some 50 right-wing protesters burned a Catalan independence 'estelada' flag outside the government delegation in Barcelona.
In their report, international observers criticized the lack of an official census for the poll, while the low number of polling stations also came under fire.
They also expressed concerns about the lack of privacy at polling stations when it came to voting.
The observers went on to note they hoped Catalans could vote on this issue in the future without the "challenges" faced on Sunday.
Catalan president Artur Mas has described Sunday's vote as "a complete success" and "a lesson in democracy" while Spain's new justice minister described the exercise as "sterile" and "useless".
On Monday, the Secretary-General of Spain's ruling Popular Party María Dolores de Cospedal said the vote had been "on the margins of legality" and that the results lacked validity. She added the government in Madrid would act against the vote, but did not specify how.
Mas has said his government would now push to hold an official referendum and would seek international support to help convince the Spanish government to let it go ahead.