In the Roman Catholic church, the person has to be dead to be declared a saint.
Romero, who was the Archbishop of San Salvador in South America was assassinated in March 1980, a day after he had called on the military to stop killing innocent civilians in El Salvador's civil war.
More than 100,000 people attended his funeral and the military fired on the crowd killing dozens of mourners.
Archbishop Romero was one of the main proponents of liberation theology which said caring for the poor meant political action to change oppressive systems in society.
His canonisation has been resisted by conservatives in the Catholic Church because of their opposiiton to liberation theology but Pope Francis made it clear soon after he became Pope that he would declare Oscar Romero a saint.
Romero was beatified on the way to becoming a Saint in a ceremony in Sal Salvador in 2015 attended by 250,000 people.
He is one of six people to be declared saints in the ceremony in St. Peter's Square on Sunday including Pope Paul VI, who oversaw the sweeping reforms of the Catholic church in the 1960s known as Vatican II.
Political divisions in El Salvador and therefore opposing views of Oscar Romero remain.
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