Thousands of people streamed through the streets of Bilbao on Saturday, demanding that inmates linked to the terror organisation ETA be allowed to serve their time in Basque jails.
They are currently detained across Spain and in France.
It was the first annual march since the conservative Popular Party returned to power in Spain after winning November’s election.
Protest leader Oskar Matute said he hoped the impact of the rally would spread beyond the Basque Country to Madrid.
“We want the Popular Party to take note and for it to decide to become an active player in political normalisation,” he said. “It must use its power in government and not become an obstacle as it has always been before.”
This was also the first such gathering since the Basque separatist group, weakened by hundreds of arrests, said in October that it was ending its armed campaign.
ETA has killed more than 800 people in its fight for an independent Basque state in northern Spain and southwestern France.