An estimated 100,000 protesters marched through the streets of Bilbao in northern Spain on Saturday in support of imprisoned members of ETA, in defiance of a Madrid court ban.
The march was initially organised by supporters who want the prisoners moved to jails within the Basque region.
The BBC reports ETA supporters have been emboldened by a recent ruling by the European Court of Human Rights, which found Spain had acted illegally by preventing their early release.
Many of the prisoners had been convicted of murder.
ETA declared an end to its armed campaign in 2011 after being blamed for the deaths of more than 800 people over four decades.
However, the BBC reports successive governments in Madrid have refused to negotiate.
About 600 ETA members are still in prison in Spain.