By John Bingham and Nick Squires in Vatican City
(The Telegraph) "Thank you for your welcome, and for your prayers," the first pope from Latin America said from a window of the papal apartment high above the square. "Pray for me," he added.
Dozens of flags from Francis's native Argentina were waving in the crowd as the former cardinal Jorge Mario Bergoglio recited the traditional Sunday Angelus prayer, the first of his papacy.
The occasion has traditionally been a moment to comment on international issues, but Francis instead used the occasion to emphasise his Italian roots.
The former Buenos Aires archbishop, whose father emigrated from Italy's northwestern Piedmont region, said he chose to name himself after St Francis of Assisi because of his "spiritual ties with this land". Earlier the pontiff was treated like a rock star on a visit to a parish church.
Around 1,000 people thronged a narrow passageway outside the Church of Saint Anna, his local parish church just inside the Vatican gates, as he arrived for mass.
In dramatic contrast with the reserved style of his predecessors, he walked along a hastily constructed barrier reaching deep into the crowd, shaking hands, laughing and joking.
It is the first time he has had a chance to meet members of the public up close since being elected on Wednesday.
There were chants of "Francesco, Francesco" as he turned and walked through the iron gates out onto the main street, where most of the crowd were waiting, leaving his anxious security men rushing to keep up.
When two clerics were brought up and introduced to him, attempting to drop to their knees, he hurriedly ushered them back onto their feet.
"He touched me, he touched me!" said one French woman holding her hand aloft.
"We just came for the weekend we never expected to meet the Pope."
As the service began, he was led inside the building, which is cloaked in scaffolding, waving as he went.
"To me, I say this humbly, the strongest message of the Lord is mercy," he said. "The Lord never gets tired of forgiving."
The mass led by the first Latin American pontiff was held in the Santa Anna Church within the Vatican walls ahead of his first appearance in a window of the papal apartments at noon (1100 GMT).
The delivery of the traditional Angelus prayer, followed by remarks expected to touch on international issues, will be the pope's second appearance before the general public since his surprise election on Wednesday.
But to the obvious surprise of the onlookers, who were starting to leave, he reappeared minutes later, wearing a purple bishop's mitre and robes, as part of the procession at the start of the mass.
Instead of simply processing up the aisle, they diverted out into the street. Initial shrieks of surprise were quickly hushed as the crowd recognised they were part of the service.
Maria Hakolinen, who prays at the church every morning at 7am, said she had never seen Pope Benedict there on a Sunday morning.
"I come here every day so I thought of course I should come to say hello you are welcome," she said.
"I see that he is a very natural, very sensitive and very special person," she said.
"We really took him into our hearts in that same moment when he prayed the Our Father in St Peter's square and started to pray with us and asked us to pray with him."
Later this morning he will give his first Sunday Angelus address in front of an expected 200,000 people.