The posters, written in Catalan, described six journalists as “information terrorists in the service of Ibex”, the stock market index of Spain's principal exchange.
Sanchez on Twitter voiced his “strong condemnation and support for the Catalan journalists targeted”.
La libertad de información es un derecho fundamental, un pilar básico e irrenunciable de la democracia que debe ser respetado y garantizado.
Nuestra rotunda condena a estas amenazas y todo nuestro apoyo a los periodistas catalanes que las están sufriendo. https://t.co/HJKV0EyrEJ
— Pedro Sánchez (@sanchezcastejon) November 6, 2019
Pablo Casado, who heads the rightwing opposition People's Party, also denounced the posters as “intolerable intimidation”.
In a statement, the Federation of Spanish Journalists' Associations and the Press Association of Madrid said “such a degree of violence and hostility towards the press had never been seen before in Catalonia”.
The appearance of the posters comes amid tensions over long sentences handed down to nine Catalan separatist leaders last month.
The sentencing triggered a huge wave of protest with days of violent clashes between demonstrators and police.
The posters come as Spain gears up for yet another general election — its fourth in as many years — that will take place on Sunday.