Thousands of people from the opposition Popular Party marched in Madrid's Plaza Colon against the Basque nationalist party Sortu, which is the outlawed political wing of the armed separatist group ETA, to protest its participation in the upcoming local election.
Before the march began, protesters began chanting, "Rubalcaba, go to jail," in reference to Prime Minister Alfredo Perez Rubalcaba, El Pais reported.
Some say they believe Rubalcaba was involved in a tip-off to ETA about plans to arrest its collaborators while negotiating with the terrorist group in 2006.
Some opposition leaders were visibly uncomfortable with the tone of the march as protesters voiced concerns the government is still in secret talks with ETA.
Madrid Mayor Alberto Ruiz-Gallardon and Popular Party Secretary-General Maria Dolores de Cospedal appeared at the march, but refused to comment on the issue.
The courts are investigating the case, known as the Faisan.