Pope Leo said in a Christmas Eve sermon on Wednesday that the story of Jesus being born in a stable because there was no room at an inn should remind Christians that refusing to help the poor and strangers today is tantamount to rejecting God himself.

Leo, who has made care for immigrants and the poor key themes of his early papacy, said Jesus' birth showed God's presence in every person as the pontiff led the world's 1.4 billion Catholics into Christmas at a Mass in St. Peter's Basilica.

"On earth, there is no room for God if there is no room for the human person. To refuse one is to refuse the other," said the pope during the solemn service, attended by about 6,000 inside the basilica.

Leo, the first US-born pope, is celebrating his first Christmas after being elected in May by the world's cardinals to succeed the late Pope Francis.

The pope, who has criticized US President Donald Trump's divisive immigrant crackdown, quoted a line from the late Pope Benedict XVI lamenting that the world does not care for children, the poor or foreigners.

"While a distorted economy leads us to treat human beings as mere merchandise, God becomes like us, revealing the infinite dignity of every person," said Leo.

"Where there is room for the human person, there is room for God," he said. "Even a stable can become more sacred than a temple."

Outside the basilica, about 5,000 people watched the service on screens from St. Peter's Square, holding umbrellas and wearing ponchos under a hard rain in Rome.

Leo, 70, came outside to greet them before the start of the Mass. "I admire and respect and thank you for your courage and your wanting to be here this evening," he said, "even in this weather."

On Thursday, the pope will celebrate a Christmas Day Mass and deliver a twice-yearly "Urbi et Orbi" (to the city and the world) message and blessing.