The group of 70 recently released former members of the militant Basque group ETA came together to make the announcement, which was seen by many as a guarded apology, the BBC reported Saturday.
The Spanish government dismissed the gesture, calling for the group's dissolution.
ETA members from the 1970s to the 1990s claimed responsibility for numerous bombings and targeted killings -- more than 800 people died as a result, the Spanish government says -- while fighting for an independent state for their ethnic group in northern Spain and southern France.
A European human rights court ruled recently members of ETA should be released. The Spanish government harshly criticized the decision but has since begun to comply.
While accepting responsibility for the terrorist acts, ETA members called on the Spanish government to release the remaining members still in prison, the BBC said.
ETA said in 2011 it had ended its use of violence in pursuit of a Basque state.