An independence referendum, later declared illegal by the Constitutional Court of Spain, takes place in Catalonia.

An independence referendum was held on 1 October 2017 in the Spanish autonomous community of Catalonia, passed by the Parliament of Catalonia as the Law on the Referendum on Self-determination of Catalonia and called by the Generalitat de Catalunya. The referendum, known in the Spanish media by the numeronym 1-O, was declared unconstitutional on 7 September 2017 and suspended by the Constitutional Court of Spain after a request from the Spanish government, who declared it a breach of the Spanish Constitution. Additionally, in early September the High Court of Justice of Catalonia had issued orders to the police to try to prevent the unconstitutional referendum, including the detention of various persons responsible for its preparation. Due to alleged irregularities during the voting process, as well as the use of force by the National Police Corps and Civil Guard, international observers invited by the Generalitat declared that the referendum failed to meet the minimum international standards for elections.