Pilgrims from across the globe are expected to flock to Saint Peter's Square to pay tribute to two men hailed by Francis for their courage in turbulent times and their dedication to social justice and the poor.
The pair will be made saints along with five others, including an Italian youth who died of bone cancer aged 19 and a German nun.
The canonisations come just days before the International Day for the Eradication of Poverty on October 17.
Described as a simple man close to the downtrodden, Romero stood up for peasant rights in the face of a right-wing backlash which painted him as a radical supporter of “liberation” theology in his small, impoverished central American nation.
Paul VI was the first head of the Roman Catholic Church to attempt to reform the Vatican's powerful and unruly Curia, the first to hold weekly general audiences with the common man in Saint Peter's Square, and seek the opinions of non believers.
He also was famously the first to reject the papal trappings of luxury, setting aside the traditional jewel-encrusted tiara shortly after his election in 1963 and donating its value to the poor — a gesture echoed by Francis, who renounced the papal apartment and gold cross.
Paul VI supported Romero, telling him at a meeting in June 1978 to “proceed with courage” in his defence of human rights. Just two years later, the latter would be shot in the heart during a service at the start of a bloody civil war which claimed some 75,000 lives.
A woman walking past a mural depicting slain Salvadorian Archbishop Oscar Romero at La Divina Providencia Hospital in San Salvador on March 17, 2010. Photo: Jose Cabezas/AFP
Efforts to recognise Romero met with heavy opposition from conservative Catholics and the Salvadoran right, who saw veiled Marxism in his sermons eulogising the poor and radio broadcasts condemning government repression.
But Francis — the first Latin American pope — beatified him as a “martyr” in 2015 to popular acclaim after his predecessor, the retired Benedict XVI, championed his cause for canonisation.
It was Benedict who also extolled Paul VI, a man he once described as “almost superhuman”.
The softly-spoken Giovanni Battista Montini was elected pope in 1963 in a difficult period for the church, which lost many believers as populist rebellions swept across the West.
He completed the reforms of the Second Vatican Council and was the first pilgrim pope, crossing continents on his trips to meet the faithful.
At his beatification mass, Francis had hailed him a “brave Christian”.
Pope Paul VI leaves the Basilica of the Annunciation in Nazareth after celebrating a mass during his visit to Jordan, Israel, Jerusalem and the Palestinian territories. Photo: Stringer/EPU Files/AFP
But he was also hampered by a reputation for being weak and overly cautious, and he was uneasy with the liberation of morals of the time.
He is most famous for reaffirming the church's ban on contraception — despite the fact that his own advisory commission voted overwhelming to lift the prohibition.
The decision enraged many Catholics at a time of sexual freedoms, with women demanding the right to use the birth control pill.
Sunday's ceremony has been timed to coincide with a meeting of the world's bishops, for it was Paul VI who introduced the tradition of holding such synods.
Francis has chosen to canonise Montini during a meeting dedicated to youth.
Paul VI felt close to the church's young believers, though he now appears somewhat stern compared to selfie-loving and social media-friendly Francis.
Youth, he warned at the beatification of teenager Nunzio Sulprizio, one of Sunday's other new saints, “should not be considered the age of free passions, of inevitable falls, of invincible crises, of decadent pessimism, of harmful selfishness”.