Tens of thousands of people marched through the streets of northern Bilbao calling for an amnesty that would allow Eta prisoners to serve out the remainder of their sentences in their native Basque region rather than in jails further afield.
Around 100,000 people marched after a National Court judge gave permission for the demonstration to take place.
Organisers had called for the march to be silent but participants chanted prisoners' names loudly.
Eta has killed 829 people since the late 1960s in bombings and shootings to force the creation of a Basque homeland in northern Spain and southwest France.
Eta is considered a terrorist organisation by Spain, the US and the European Union. The march took place in a peaceful atmosphere, police said.