Hundreds of thousands gathered in St. Peter's Square on Sunday, a day after Pope Francis' funeral, for another ceremony to honor him on the second of nine official days of mourning for the global Catholic Church.

But instead of world leaders like U.S. President Donald Trump, who attended the funeral, the vast square and surrounding streets were filled with what Italian police estimated were 200,000 mainly young people, many wearing scouting uniforms or colorful T-shirts.

Sent as delegations from across Italy and several other countries, they had planned to come to Rome for a now-postponed ceremony to proclaim the first Catholic saint from the millennial generation.

Instead of honoring Carlo Acutis, an Italian boy who died from leukemia aged 15 in 2006, they were now honoring Francis, who died at the age of 88 on April 21 after 12 years leading the 1.4-billion-member Church.

"Even having to change plans, it is a joy to remember (Francis)," said Samuele Arregetti, an 18-year-old who had come from Bergamo in northern Italy for the Mass.

"We are very sad for his death but now thinking of him we are also happy ... that he is in Heaven."

Ahead of Sunday's service, the young crowds pressed down the main boulevard through Rome towards the Vatican. Shouts of "viva Francesco" (long live Francis) and "il gioventu del papa" (the pope's youth) were heard.

Cardinal Pietro Parolin, the Vatican's secretary of state, led the ceremony. In his sermon, he told the young people present that Francis would have wanted "to meet you, to look into your eyes, and to pass among you to greet you."

VISITORS FLOCK TO POPE'S TOMB

But Parolin, considered a leading contender in the coming papal conclave, did not offer much of his own vision for the future of the Church. His sermon was short, about 11 minutes, and stressed some of the central themes of Francis' papacy.

Francis was buried on Saturday at Rome's Basilica of St. Mary Major. The first public visits to his tomb – which has just "Franciscus," his name in Latin, inscribed on the top – started on Sunday morning.

Thousands have already visited the church, with police urging visitors to leave as soon as soon they have seen the tomb, to help keep the long queue moving.

Attention is now switching to who might succeed Francis.

The secretive conclave is unlikely to begin before May 6, and might not start for several days after that, giving cardinals time to hold regular meetings beforehand to sum each other up and assess the state of the Church, beset by financial problems and ideological divisions.

The next meeting of cardinals, known as a "general congregation," will be held on Monday morning, when they could set the date for the conclave.