Protesters in the Basque country, who hold this demonstration every January, held up placards of a finger pointing to the sky as they denounced the prison conditions endured by Basque separatists.
Almost all members of Eta, a group that gave up armed resistence to Spain in 2011, have been jailed but the movement has refused to disband.
They have been calling for an amnesty for their incarcerated members so they can be reunited with their families.
Basque newspaper Gara estimated the number of people in the Bilbao streets on a rainy day as 78,000, while police declined to give a crowd estimate.
An association defending the rights of Eta prisoners released a statement blasting "those who keep in prison prisoners who have long served their sentences imposed by the courts".
It also demanded that the prisoners be transferred to jails closer to their families.
The collective of Basque political prisoners (EPPK) recently gave up its demand for universal amnesty but has asked for negotiations over suspending sentences on a case by case basis.
According to prisoners' families association Etxerat, in August 2016, some 279 Eta members were incarcerated in Spain with another 79 in France.
Eta has been blamed for the deaths of 829 people in bombings in Spain since its creation in 1959.
It wants to create an independent homeland from the Basque regions in northern Spain and southwest France.