Hackers from the hacktivist group Anonymous targeted websites run by Spain's Ministry of Public Works and Transport in support of the Catalan independence movement.

The attacks were part of an ongoing dispute that has played out both online and in the streets over Catalan's desire for independence. Some sites were defaced to display a "Free Catalonia" slogan, while others were bombarded with DDoS attacks.

"In the name of all the Catalan independence and democracy, Anonymous Catalonia asks all the Anons of the world who are in favour of the freedom of expression [...] and peaceful dialogue to persist in the #FreeCatalonia operation until 29 October 2017,” the group tweeted from several of its accounts during and in the days leading up to the attacks.

"Hacktivist group Anonymous, through associated Twitter accounts, is announcing a massive cyber-attack for tomorrow,” said the Spanish government's Departamento de Seguridad Nacional said on its site. “In the last few weeks, state websites have received different cyber-attacks under the same slogans," the Spanish government's Departamento de Seguridad Nacional (Department of National Security) said in a statement.

Despite having prior knowledge, the attacks appear to be have been successful. 

Anonymous was not alone in joining the fray. Other as yet unknown hackers countered by targeting pro-independence sites earlier this month with DDoS attacks and also sent spam bombardments to disable an email account used by Catalonian mayors to coordinate the vote, according to the Washington Post.

DDoS attacks are becoming a common tool to disrupt business or access sensitive corporate information Corero Network Security Vice President Stephanie Weagle said.