Benedict XVI hit out at what he described as the "increasing marginalisation of religion" during a dramatic second day of the papal visit during which police arrested six men for an alleged terrorist plot to attack the Pope.
Speaking to members of civil society in Westminster Hall, a venue filled with symbolism for the Catholic Church as the place where St Thomas More was condemned to death for refusing to abandon his loyalty to Rome, the Pope last night praised Britain's Parliamentary tradition for creating a "pluralist democracy which places great value on freedom of speech".
But he also attacked moves to relegate religion to the private sphere and said more should be done to protect religious festivals such as Christmas.
His comments came at the end of a hectic day which included a warning to children against the allure of "celebrity culture", a pointed discourse on the limits of science to explain human existence, and a historic meeting with the Archbishop of Canterbury.
Standing in Westminster Hall in front of four former prime ministers – David Cameron was absent for his father's funeral – the 83-year-old pontiff railed against secular intolerance of religious belief, a key theme of his four-day visit.
"I cannot but voice my concern at the increasing marginalisation of religion, particularly of Christianity, that is taking place in some quarters, even in nations which place a great emphasis on tolerance," he said. "There are those who argue that the public celebration of festivals such as Christmas should be discouraged, in the questionable belief that it might somehow offend those of other religions or none. And there are those who argue – paradoxically with the intention of eliminating discrimination – that Christians in public roles should be required at times to act against their conscience."
His speech, billed by Catholic officials as the most important of the visit, is part of a Vatican fightback against the kind of secularism in Western societies that the Pope believes threatens the rights of religious people to practise their faith openly, such as the right for Catholic adoption agencies to refuse their services to homosexual couples.
It was a sober and critical ending to a day that had begun very differently with a dawn raid by anti-terror police on a rubbish depot in central London, where five street sweepers were arrested. A sixth man was taken into custody later in the day.
The men, some of whom are thought to be Algerian, were held on suspicion of the commission, preparation or instigation of acts of terrorism. They all worked for Veolia Environment Services, a company contracted by Westminster Council to keep the streets of central London free of litter.
No materials or explosives were found at the site of the arrests which were reportedly sparked after an overnight tip-off. The men were being questioned last night and have not been charged.
Federico Lombardi, the Vatican's spokesperson, said the Pope was informed of the threat during his visit to St Mary's University College in Twickenham where 4,000 school children had gathered to greet the Pope to kick off the second day of his visit.
Apart from the school uniforms it could have been mistaken for a pop concert. Thousands of excitable children crammed into an open-air stadium waiting for the headline act.
When the Pope came into view, aboard his Popemobile, he was given the kind of greeting normally reserved for rock stars as thousands of children burst into excited screams.
"That Popemobile was so big," gasped nine-year-old Theresa Mortlock, a pupil from St Elizabeth's primary school in west London. "To see him in the flesh was amazing, I nearly fainted. My nan is so jealous".
Using a far more informal tone compared to his attacks on "aggressive secularism" the day before, the Pope warned children of the pitfalls of our "celebrity culture".
"I hope that among you listening to me today there are some of the future saints of the 21st century," he said. "We live in a celebrity culture and young people are often encouraged to model themselves on figures from the world of sport and entertainment.
"Being highly skilled in some activity or profession is good, but it will not satisfy us unless we aim for something greater still. It might make us famous, but it will not make us happy."
In a meeting with Jewish, Muslim, Sikh and Hindu leaders shortly after midday, his attention had turned to the sciences. "They cannot satisfy the deepest longings of the human heart," he said. "They cannot fully explain to us our origin and our destiny, why and for what purpose we exist."
He did not mention it specifically but his comments came a week after renowned theoretical physicist Stephen Hawking concluded that God was not needed to create the universe.
In the same speech the Pope called on other faiths to encourage free worship, especially in countries where religious minorities are persecuted.
The spirit of ecumenicalism continued as the Papal entourage met Archbishop Rowan Williams, the leader of the Anglican Church. The historic occasion marked the first time a Pope had set foot inside Lambeth Palace, the seat of English Catholicism before the Reformation. It comes at a time when the relationship between the two churches is decidedly frosty with senior Anglicans furious at the Vatican's recent attempts to woo disaffected members who are opposed to the consecration of gay and women clergy.
The Pope was keen to avoid dwelling on such diplomatic difficulties. "It is not my intention today to speak of the difficulties that the ecumenical path has encountered and continues to encounter," he said. "Those difficulties are well known to everyone here."
Instead he gave thanks for "the deep friendship" between the two faiths.
What the Pope said
St Mary's University College, Twickenham
"I wish to add a particular word of appreciation for those whose task it is to ensure that our schools provide a safe environment for children and young people. Our responsibility towards those entrusted to us for their Christian formation demands nothing less.
Westminster Hall, London
"I cannot but voice my concern at the increasing marginalisation of religion, particularly of Christianity, that is taking place ... even in nations which place a great emphasis on tolerance."
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies