Many Basque political leaders were present at the march in Bilbao, carrying banners saying "The Basque region wants solutions," a reference to a long espoused yearning for greater regional autonomy.
Some demonstrators could be heard shouting for a return of ETA prisoners to jails in the Basque region.
Among speakers at the rally was Pello Urizar, secretary general of party Eusko Alkatasuna, who called on Spanish leaders to commit themselves to a political solution to regional tensions like the British did to end separatist violence in Northern Ireland.
Although ETA, classified as a terrorist organization by Spain, the U.S. and the European Union, has declared two previous "permanent" cease-fires, many participants and observers said they thought this time they meant it.
"It's final, this is it, there is not going to be any more violence or terrorist attacks. ETA is finished," said Manuel Escudero, 63, head of the Basque region's Deusto university business school.
ETA's political supporters renounced violence last year in a much-debated rift with the past and called for the whole pro-independence movement to shift to a strictly peaceful agenda.
ETA attacks have left 829 people dead after four decades of shootings and bombings.
Now ETA has called for talks with Spain and France — the homeland ETA wants to create includes parts of southwest France — on the "consequences" of the conflict, a reference to an estimated 700 ETA prisoners held in jails dotted around Spanish and in France.
Many demonstrators Saturday clearly sympathized with ETA's request for some sort of concession which might move prisoners to the Basque region, allowing them to be closer to their families.
Dani Maeztu, leader of Basque Socialist party Alalar, said it was also time to begin to think about "reparations toward the victims of terrorism," almost certainly the first time this thorny subject has been aired at a separatist rally.