LISBON: One of the biggest crowds ever seen in Portugal mobbed Pope Francis on Thursday as he rode in an open vehicle to a rally where he told an ocean of young Catholics from all over the world to beware the false happiness lurking in social media.

Francis was driven along the streets of the Portuguese capital in a white "popemobile" past people waving the national flags of scores of countries, converging on the sprawling Edward VII park.

The crowd, which police said numbered about half a million, was the largest in Lisbon since celebrations in 2016 when Portugal's men won the European soccer championships.

"It's incredible because Portugal is such a small country that it seems that nobody knows about it, so it's good to know that we are hosting an event of this importance," said Mariana Moreira, 20, from the northern city of Barcelos. "We are happy."

Thursday's early evening event was the first of several with the pope for World Youth Day, a gathering that takes place every three years in a different city. The event, which opened on Tuesday, will close with a papal Mass on Sunday.

Seemingly energised by the crowd of young people singing and dancing, Francis warned them to beware "the illusions of the virtual world" where algorithms used their names for market research but could never understand a person's uniqueness.

"Many realities that attract us and promise happiness are later shown to be what they really are: vain, superfluous and surrogate things that leave us empty inside," he said.

GOD OVER GOOGLE

Francis said God was "not a search engine", and did not give simple, straightforward answers, but responses that prompted a healthy restlessness and encouraged people to seek the truth.

The festival is taking place in the shadow of Portugal's version of the Church's global sex abuse crisis, following the publication of a report six months ago that said at least 4,815 minors had been sexually abused by clergy over seven decades.

One of the young people who addressed the pope before he spoke asked him "to put things right in the Church for a better future". Francis met with 13 victims of clergy sexual abuse on Wednesday night.

Since the start of his papacy, Francis has been trying to make the Church more welcoming and less condemning, including to members of the LGBT community, but without changing teachings that urge those with same-sex attraction to be chaste.

He has pushed a series of reforms, including giving more roles to women, but faces a delicate balance between appealing to more liberal believers and upsetting conservatives.

He told the crowd the Church had room for everyone, "including those who make mistakes, who fall or struggle", and led them in a chant of "Todos, todos, todos!" (Everyone, everyone, everyone!).