London-born teenager declared a saint by Pope Leo

A London-born Italian teenage computer whiz who died in 2006 aged 15 has been declared the Catholic church's first millennial saint during an open-air mass in a packed St Peter's Square.

- Kate Connolly and Angela Giuffrida Rome

Carlo Acutis, who died of leukaemia, built multilingual websites to spread Catholic teaching, later earning him the nickname "God's influencer". He was canonised by Pope Leo XIV alongside Pier Giorgio Frassati, another young Catholic activist, who died a century ago.

An hour before the mass, St Peter's Square had filled with tens of thousands of pilgrims, many of them millennial Italians and Americans, as Acutis's family looked on.

The crowd numbering an estimated 80,000 braved sweltering heat and many were forced to spill out into the boulevards next to the square, with eyewitnesses describing the atmosphere as joyous and party-like.

"The greatest risk in life is to waste it outside of God's plan," the pope said in his homily, adding both Acutis and Frassati had made "masterpieces" of their lives by dedicating them to God.

The new saints "are an invitation to all of us, especially young people, not to squander our lives, but to direct them upwards," he said.

Both ceremonies had been scheduled for earlier this year, but were postponed after the death of Pope Francis, who had fervently pushed Acutis's sainthood case forward to attract young Catholics to the faith.