They declared the secession referendum illegal, and police blocked polling stations.
12 Catalan leaders who organised the vote have gone on trial in Madrid, in what their supporters are calling a 'show trial'.
Spain's Former Prime Minister Mariano Rajoy is among 500 witnesses expected to testify live on TV over the next three months.
The leaders face up to 25 years in jail if convicted of rebellion, sedition, and misusing public funds.
Will divisions in Spain be widened?
Presenter: Elizabeth Puranam
Kristian Herbolzheimer - Director, International Catalan Institute for Peace
Victor Sole - International Officer, Nationalist Youth of Catalonia
Gabriel Garroum - Researcher on Catalan nationalism, King's College London
Subscribe to our channel http://bit.ly/AJSubscribe
Follow us on Twitter https://twitter.com/AJEnglish
Find us on Facebook https://www.facebook.com/aljazeera
Check our website: http://www.aljazeera.com/