The former leader of the PSOE party, Alfredo Pérez Rubalcaba, died on Friday in a Madrid hospital after suffering a stroke at his home on Wednesday. He was 67.
Rubalcaba led the party in opposition from 2012-2014, having previously served as minister in the Socialist governments of both Felipe González and José Luis Rodríguez Zapatero.
He was described on Friday by acting prime minister Pedro Sánchez as a "great statesman".
He has been praised for his role in bringing an end to the violence of Basque terrorist group ETA when he was minister under Rodríguez Zapatero.
He recently turned down the party's candidacy for Mayor of Madrid in this month's elections.
Pérez Rubalcaba retired from the political frontline in 2014 when he returned to his job as a Chemistry lecturer at Madrid's Complutense University.