Critics of what Spanish media have described as "the biggest ever beatification in the history of the Church" accuse the Church of helping to legitimise Francisco Franco's military uprising and dictatorship.
An alliance of Spanish groups wrote to Francis asking him to cancel a "political act of pro-Franco affirmation" by the Catholic Church.
The letter says the Church had "forgotten the victims of Francoist repression".
Some 25,000 people are expected to attend the ceremony of 522 "martyrs of the 20th century in Spain" in the eastern city of Tarragona, according to the Spanish Episcopal Conference.
Pope Francis will open the ceremony with a pre-recorded video message.