The beatification of Archbishop Oscar Romero is expected to draw 300,000 people tomorrow.
The Mass will take place in a square in San Salvador, the capital of El Salvador, at 10am (4pm GMT).
The Pope’s representative, Cardinal Angelo Amato, prefect of the Congregation for the Causes of Saints, will read out the decree of beatification.
Seven cardinals, 90 bishops and more than 1,000 priests are expected to attend, along with several heads of state, including the presidents of Ecuador and Panama.
Ceremonies will begin in San Salvador this evening, with a torchlit procession, an all-night vigil and a Mass celebrated by Cardinal Oscar Rodriguez Maradiaga, chairman of the Council of Cardinals.
Archbishop Romero was shot dead while celebrating Mass in a hospital chapel 35 years ago.