“The Spanish state is a prison for the people, and this is shown by denying the national identity of the Catalan countries,” ETA said in a statement published in the Basque newspaper Gara.
“The Spanish state has also become a prison for democracy, since it has trampled on the rights of the Catalans.”
The wealthy northeastern region of Catalonia is scheduled to hold an independence referendum on October 1 despite a crackdown by Madrid which wants to prevent a vote deemed unconstitutional by the courts.
Spanish security forces on Tuesday began working to “neutralise” polling stations for the vote.
ETA was blamed for the deaths of 829 people in a decades-long campaign of violence for an independent state in the Basque region, which straddles the Spanish-French border.
The group carried out its last attack in 2010 and disarmed in April this year.
Some former ETA members have joined a Franco-Spanish Basque political party called Sortu that is working for “full freedom” for the region's 2.2 million people.
The regional leader of Spain's Basque Country, Inigo Urkullu, urged Madrid on Sunday to allow independence referendums like those held in Scotland and Quebec.
ETA said in its statement Wednesday that “civil and political rights are again at issue”.
“And in order to violate them” the government in Madrid has “not needed to use the pretext of armed struggle. It has been shown that the notion that 'without violence everything is possible' was totally false.”