Pope Benedict XVI has celebrated an open-air Mass in Glasgow, attended by tens of thousands of people, on the first day of his visit to Britain.
About 70,000 people attended the celebratory event in Bellahouston Park.
In his homily, the Pope warned against people who seek "to exclude religious belief from public discourse".
Earlier, he urged the UK to resist "more aggressive forms of secularism". The Pope has now arrived by plane at Heathrow.
He was met by London Mayor Boris Johnson, who presented him with three books, including Mr Johnson's own historical book, To Dream of Rome.
The Pope is staying the night at the Apostolic Nunciature - the residence of his representative in Britain - in Wimbledon, south-west London, ahead of a day of events, including a service at Westminster Abbey, on Friday.
A spokesman for the Catholic Church in England and Wales hailed the first day of the visit as a "success".
"Everybody in the Pope's entourage was overwhelmed by the people on the streets of Edinburgh and the turn out in Glasgow this evening. It wasn't just the size of the crowd but their enthusiasm," he said.
In his sermon in Glasgow, the Pope said that those individuals who wished to exclude religion even went as far as painting it "as a threat to equality and liberty".
But he insisted: "Religion is in fact a guarantee of authentic liberty and respect."
BBC correspondent David Willey said this was the theme the Pope "intends to hammer home during his four-day state visit".
The pontiff also said that a "dictatorship of relativism" threatened "to obscure the unchanging truth about man's nature, his destiny and his ultimate good".
He added: "Society today needs clear voices which propose our right to live, not in a jungle of self-destructive and arbitrary freedoms, but in a society which works for the true welfare of its citizens and offers them guidance and protection in the face of their weakness and fragility."
He also urged the young Catholics of Scotland to resist the "destructive and divisive" temptations of drugs, money, sex, pornography and alcohol.
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.
The Pope travelled to Glasgow from Edinburgh, where he was welcomed on his visit to the UK by the Queen at Holyroodhouse.
On his arrival at Bellahouston Park, 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.
Britain's Got Talent star Susan Boyle sang the hymn How Great Thou Art after the sermon.
She had 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, also performed.
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 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.
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.