Hundreds of people on Tuesday joined the families of the 21 victims died in a 1987 bombing in the northeastern city of Barcelona to mark its 20th anniversary.
The car bomb in a department store parking lot on June 19, 1987 killed 21 and injured 45.
In a ceremony in downtown Barcelona near the site of the blast, authorities placed a wreath at the foot of a monument remembering the victims and musicians played pieces by Catalan cello master Pablo Casals.
Speaking at the ceremony, Interior Minister Alfredo Perez Rubalcaba stressed that the Spanish government will not give in to terrorism.
ETA, whose name is a Basque-language acronym for Basque Homeland Freedom, has killed more than 800 people since 1968 in its campaign for a separate Basque state. It is classified as a terrorist group by the United States and the European Union.
ETA called off a 15-month cease fire with the Spanish government last week, blaming it for ruining the peace process and saying it will be "active on all fronts," a threat interpreted as meaning it would resume its attacks.