The Pope has expressed fears for Christianity's future as a string of historic visits went ahead despite six arrests over an alleged plot against him.
Benedict XVI warned of the faith's "increasing marginalisation" on a day which saw him make first visits by a pontiff to Lambeth Palace and Westminster Abbey.
As crowds of supporters and thousands of protesters crammed the streets of Westminster to catch a glimpse of the religious leader, he repeatedly argued religion should be recognised for its "vital" contribution to the nation. He told MPs, peers and religious leaders in Westminster Hall that there were "worrying signs" of a failure to appreciate the rights of believers to freedom of conscience and the "legitimate" role of religion in public life - and said a moral failure was to blame for the global financial crisis.
His comments came after a security scare surrounding his arrival in London. The Pope was informed of six arrests in connection with a suspected plot against him but remained "totally calm", his spokesman said.
On a number of occasions during the Popemobile's journeys through Westminster, he blessed babies which were taken from the crowd by security officers. But while some chanted "Benedict, we love you", critics booed and jeered.
He ended his tour at Westminster Abbey where he publicly shook hands with a clergywoman, the Rev Dr Jane Hedges, for the first time.
During his address at the Abbey service, which he led alongside the Archbishop of Canterbury, Rowan Williams, the Pope spoke of the need for Christians to talk about their faith even though society had become "increasingly indifferent or even hostile".
It was one of numerous references to society's move away from Christian heritage. In his speech at Westminster Hall, where audience members included former prime ministers Baroness Thatcher, Sir John Major, Tony Blair and Gordon Brown, the Pope said: "There are those who would advocate that the voice of religion be silenced, or least relegated to the purely private sphere. 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."
The Pontiff had earlier been warmly greeted by the Archbishop of Canterbury at Lambeth Palace. The Archbishop told the Pope: "You are here for another historic meeting and you are most welcome."
The Pope began the day with a reference to the child abuse scandal, saying schools must provide a "safe environment" for children. Speaking at the Chapel of St Mary's University College in Twickenham, south-west London, he said: "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."