The Queen has led a host of British leaders in congratulating the newly-elected Pope Benedict XVI.
A nun prays at Westminster Cathedral on Tuesday
Prime Minister Tony Blair and leaders of the main opposition parties have also sent their congratulations.
Although his election has been broadly welcomed, one British priest says he is "shocked" by the appointment of Cardinal Joseph Ratzinger.
Father Ray Lyons, of the Portsmouth Diocese, says the German is "not the man to act as healer and reconciler".
The new Pope, who is thought to have a conservative outlook, did not have a "reform agenda", Father Lyons said.
"He is a great thinker who will probably go along the same lines as his predecessor, but he is not what I felt was needed," he added.
Several Britons who were in St Peter's Square have told of the "electric" scenes in the Vatican on Tuesday night.
Software director John Farmer, 36, from Edinburgh, saw the white smoke appearing from the chimney of the Sistine Chapel - the signal that the cardinals had decided on the new Pope.
He said: "There was a lot of confusion because we weren't sure if it was white or black.
"When that moment finally came, it was an unbelievable sense of joy."
Kim Campbell, 16, from Dublin, said seeing Pope Benedict XVI on the Basilica's main balcony was "unbelievable".
She said: "It was so exciting to see the new Pope come out.
"Everyone applauded and cheered."
A thanksgiving service was given by Bishop George Stack at Westminster Cathedral in London a few hours after the Vatican's announcement.
Worshippers at Westminster Cathedral applauded loudly when they heard the name of their new Pope.
Archbishop Patrick Kelly led Catholics in Liverpool at a special service in the city's Metropolitan Cathedral.
Mr Blair has sent his best wishes to the Vatican, saying he looked forward to continuing co-operation with the Holy See on issues including Africa and development.
Charles Kennedy, Liberal Democrat leader, offered his "profound good wishes", while senior Tory Michael Ancram wished him "strength and courage".
The Most Rev Patrick Kelly, who has met Pope Benedict on several occasions, described him as "wise, profound and humble".
Leaders of the UK's other religious communities also welcomed the new Pope's appointment.
Archbishop of Canterbury Dr Rowan Williams, head of the Church of England, said the appointment was of "great significance to Christians everywhere".
Chief Rabbi Dr Jonathan Sacks said he hoped the new Pope would continue to work at improving relations between Christianity and Judaism.
The secretary general of the Muslim Council of Britain, Iqbal Sacranie, said also hoped for further understanding between faiths.