Thousands of people have gathered in Glasgow, where Pope Benedict XVI is celebrating an open-air Mass. About 65,000 people cheered and waved flags as the Pope arrived to organ music and hymns at Bellahouston Park.
The Pope travelled from Edinburgh, where he was welcomed on his visit to the UK by the Queen at Holyroodhouse.
Earlier, he said he wanted to "extend the hand of friendship" to the country and urged the UK to resist "more aggressive forms of secularism".
The Catholic Church in Scotland had hoped to attract up to 100,000 to the event, but later reduced the capacity after a slow take-up of tickets.
On his arrival, the Pope was given a traditional Scottish welcome from the Strathclyde Police pipe band.
As he made a circuit of the park in his Popemobile, the window of the vehicle was rolled down and the pontiff kissed a baby dressed in a pink romper suit.
First Minister Alex Salmond arrived at the park wearing a tartan scarf and a yellow flower, while crowds waved yellow and white Vatican flags.
During the service, Britain's Got Talent star Susan Boyle is due to sing the hymn How Great Thou Art.
She has already entertained cheering crowds - many of whom were waving commemorative flags and "pilgrim packs" - with the song I Dreamed A Dream. The winner of Pop Idol 2003 Michelle McManus, who is from Glasgow, has also performed.
'Special contribution'
The trip is the first to the UK by a Pontiff since John Paul II in 1982. It is also the first to be designated a state visit because the Pope has been invited by the Queen rather than the church.
Earlier, the Queen said the visit was an opportunity to "deepen the relationship" between Catholicism and the Churches of England and Scotland. She also praised the Catholic Church's "special contribution" to helping the poorest and most vulnerable around the world.
The Pope said he wanted to "extend the hand of friendship" to the entire UK, not just the Catholic population.
He added: "Today, the United Kingdom strives to be a modern and multicultural society. In this challenging enterprise, may it always maintain its respect for those traditional values and cultural expressions that more aggressive forms of secularism no longer value or even tolerate."
The tone of the Pope's speech in Edinburgh seemed to show a determination to try to understand British society better and meet criticism of his visit with reasoned argument.
He told the Queen that during both their lifetimes Britain had stood against a Nazi tyranny that wished to eradicate God from society.
The Pope referred to the situation in Northern Ireland, saying the UK government had helped give birth to a peaceful resolution of the conflict there.
"I encourage everyone involved to continue to walk courageously together on the path marked out for them towards a just and lasting peace," he said.
The Pope also took a crack at the British media, saying it needed to have a greater responsibility because its opinions reach such a wide audience.
During the rest of his visit, the Pope is expected to continue to stress, as he has done in the past, the close connection between faith and reason.
The Popemobile then joined the annual St Ninian's Day parade where, despite tight security, police estimated that about 125,000 people turned out to cheer him on.
Presbyterians, secularists, and other groups in Edinburgh had planned to protest against Vatican policies on birth control, gay rights and abortion, although police did not report any large demonstrations.
Small groups from the organisation Protest the Pope, the Grand Orange Lodge of Scotland and the founder of the Free Presbyterian Church, the Reverend Ian Paisley, now Lord Bannside, turned out in protest.
The Pope is due to leave Glasgow bound for Heathrow, arriving at 2115 BST.
Dioceses in England and Wales have reported thousands of unfilled places for a vigil in London's Hyde Park on Saturday and a beatification Mass in Birmingham on Sunday for 19th century cardinal John Henry Newman.
The visit has caused controversy in the UK because of the cost and the scandal surrounding child abuse within the Catholic Church.
SIC: BBC/UK