Óscar Romero, the Salvadoran priest who championed social justice for the poor and dispossessed, will be proclaimed a saint by Pope Francis in a canonisation ceremony in Rome on Sunday, almost four decades after he was assassinated by a rightwing death squad.

The former archbishop of San Salvador, who was closely associated with the Latin American liberation theology movement of the 1960s and 70s, will be canonised along with six others at the ceremony in St Peter’s Square. They include Pope Paul VI, who oversaw the sweeping Vatican II reforms of the Catholic church in the 1960s.

For years, conservatives within the church sought to block Romero’s canonisation because of his association with liberation theology, a movement whose followers argued that it was not enough for the church to empathise with and care for the poor. Instead, they said, the church needed to push for political and structural changes to eradicate poverty, even – some believed – if this meant supporting armed struggle against oppressors.

Clare Dixon, head of the Catholic aid agency Cafod’s Latin America region, said: “Oscar Romero is revered in his native El Salvador. He ranks alongside the likes of Martin Luther King and Mahatma Gandhi as one of the most remarkable figures of the 20th century.

“His canonisation will give Romero the wider recognition he so richly deserves. He denounced the violence which was tearing his country apart, he spoke out against oppression, and stood against injustice alongside people living in poverty.”

Archbishop Oscar Arnulfo Romero, who was gunned down while giving mass in a San Salvador church on 24 March 1980. Photograph: AP

Father Robert Pelton, a Romero expert at the University of Notre Dame in Indiana, said the Latin American priest was a “wonderful pastoral example to the world”. He added: “I’m surprised how long it took to finally get to the truth about him.”

Francis began the process of declaring Romero a saint soon after becoming pope. About 250,000 people attended Romero’s beatification ceremony – the penultimate step towards becoming a saint – in San Salvador in May 2015.

Earlier this year, the Vatican’s Congregation for the Causes of Saints declared that a miracle had occurred following Romero’s intercession – the final step before being proclaimed a saint. A 34-year-old pregnant Salvadoran woman, who had been diagnosed as terminally ill, gave birth safely and recovered completely after prayers by her family and her church seeking Romero’s intercession.

Romero was shot through the heart by a sniper while celebrating mass in a hospital chapel on 24 March 1980, a day after he had called on the military to stop killing innocent civilians in El Salvador’s dirty war. Numerous death threats had been made against him.

At his funeral, the army opened fire, killing dozens of mourners in a crowd of more than 100,000. His murder came at the start of a 12-year civil war, in which more than 75,000 people were killed and thousands disappeared.

Dixon said that diplomatic and political pressure had been exerted on the Vatican in the years following his death to block efforts to have Romero made a saint. It was not until after the election of a leftist president in El Salvador in 2009 and Pope Francis becoming pope in 2013 that the issue of Romero’s canonisation was “pulled out of the deep-freeze”.

The former archbishop was “revered and reviled, loved and loathed” in El Salvador, “and that continues to be the case in a country which is still deeply polarised,” she said.

Nevertheless, the Catholic church would be “going absolutely crazy” this weekend celebrating the canonisation, with parties scheduled for the early hours of the morning to coincide with events in Rome.

In his diaries, Romero wrote: “Between the powerful and the wealthy, and the poor and vulnerable, who should a pastor side with? I have no doubts. A pastor should stay with his people.” His sermons, demanding social justice for poor people and excoriating politicians and military leaders, reached hundreds of thousands of people via radio broadcasts.

Francis, the first Latin American pope, was never a liberation theologian, and criticised aspects of the movement when he was a priest in Argentina, according to papal biographers. But since becoming pontiff, he has argued that the church must champion the poor and marginalised, and has repeatedly criticised capitalism and consumerism.