A teenager who died of leukaemia in 2006 became the first Catholic saint of the millennial generation on Sunday, in a Vatican ceremony led by Pope Leo and attended by thousands of young worshippers from dozens of countries.

Carlo Acutis, a British-born Italian boy who died aged 15, learned computer code to build websites to spread his faith. His story has drawn wide attention from Catholic youth, and he is now at the same level as Mother Teresa and Francis of Assisi.

Leo, the first US pontiff, canonised Acutis on Sunday along with Pier Giorgio Frassati, a young Italian man who was known for helping those in need and died of polio in the 1920s.

In impromptu remarks to crowds in St. Peter’s Square at the opening of the event, Leo said Acutis and Frassati were examples of holiness and of helping those in need. “All of you, all of us together, are called to be saints,” the pontiff told the young crowd, which had spilt out of the square down the main boulevard into the Vatican from Rome.

Acutis’ canonisation had been hotly anticipated by many Catholic youths for months. It was originally set for April but was postponed after the death of Pope Francis.

Sunday’s event is the first time that Leo, elected pope by the world’s cardinals in May, has presided over such a ceremony.

Antonio D’Averio, 24, who was at the ceremony, called the canonisation “a hand extended by the Church toward us young people.” D’Averio said he was a computer programmer and identified especially with Acutis’ story.

“He too was passionate about computer science,” said the young man. “For a saint, it’s certainly something new. It’s also something that, in my opinion, was needed.”

‘We want to follow their steps,’ said Clara Marugan Martin, aged 20, who came from Spain for the event.

“We are very pleased to be here because Carlo and Pier Giorgio are two examples of young people full of God, full of grace, and we want to follow their steps,” she said.

Being made a saint means the Church believes a person lived a holy life and is now in heaven.

Other saints who died young include Therese of Lisieux, who died at 24 in 1897 and was known for promoting a “Little Way” of charity; and Aloysius Gonzaga, who died at 23 in 1591 after caring for victims of an epidemic in Rome.

As Acutis progressed along the Church’s official path to sainthood, his body was moved to a church in the hill town of Assisi in central Italy, where St. Francis was from, in line with Acutis’ last wishes.

The new saint’s final resting place, where Acutis is entombed with a wax mould of his likeness placed over his body, wearing his track top, jeans and trainers, has become a popular devotional site, attracting thousands of worshippers every day.