Inaki Olasolo, a spokesman for the protest, said the Spanish government could take advantage of "a historic opportunity to reach a solid peace with justice, with recognition and healing" if it allowed prisoners back.
Olasolo also called for the release of those suffering from terminal illnesses, of which it is believed there are three.
Although ETA — classified as a terrorist organization by Spain, the U.S. and the European Union — has declared two previous "permanent" cease-fires, many observers think this time they mean it. Waves of arrests in recent years have repeatedly weakened its structure and diminished its ability to carry out attacks or collect funds.
However, French police late Friday arrested two people who reportedly acknowledged being ETA members and one of the suspects was in possession of a firearm.
Associated Press writer Harold Heckle in Madrid contributed to this report.