The Queen has led tributes to Pope John Paul II, who has died at the age of 84 after a prolonged illness.
He was the first Polish pontiff and the youngest of the 20th Century
She praised his "untiring efforts" in promoting peace and goodwill throughout the world".
The prime minister and the Conservative and Liberal Democrat leaders have also offered their condolences.
A number of special services - including a Requiem Mass at Westminster Cathedral - are being held at Roman Catholic churches across Britain.
The BBC's political correspondent Carolyn Quinn said the pontiff's death may affect the expected announcement of a general election on Monday.
The leader of Catholics in England and Wales, Cardinal Cormac Murphy O'Connor, told BBC News the Pope would be remembered as one of the greatest in the 2,000 years of the church.
He said he had mourned but also felt gladness.
"Here was a man who had run his course, who fought the good fight, and now his period of suffering is over.
"He has gone with confidence and hope to meet his maker."
The Palace said in a statement that the Queen remembered the Pope's work towards Christian unity.
She also singled out the pontiff's 1982 visit to Britain.
His six-day tour included a Mass at Westminster Cathedral and a meeting with the Queen at Buckingham Palace.
Prince Charles spoke of his "fond and special memories" of meeting the Pope in Rome and in Britain.
Prime Minister Tony Blair said: "Throughout a hard and often difficult life, he stood for social justice and on the side of the oppressed, whether as a young man facing the Nazi occupation in Poland or later in challenging the Communist regime. "
Conservative leader Michael Howard was one of the first British politicians to give tribute.
"In a world of change and uncertainty, people saw him as a rock - steadfast in support of freedom, unswerving in opposition to totalitarianism, robust in defence of Christian values."
Liberal Democrat leader Charles Kennedy said: "The papacy of Pope John Paul II will be remembered rightly as an historic turning point in European and world affairs."
Labour and the Tories will suspend public campaigning throughout Sunday as a mark of respect. The Liberal Democrats have no special events scheduled for Sunday.
The Pope suffered with ill health in recent years
Within minutes of the Pope's death bells started tolling at St John's Cathedral in Salford, just a few miles away from Heaton Park in Manchester, visited by John Paul II in 1982.
The Archbishop of Armagh, Dr Sean Brady, the head of the Catholic Church in Ireland, spoke of the Pope's respect for life, solidarity with people and his intellectual capacity.
He said: "He was ahead of his time with his message of global solidarity. We mourn with heavy hearts and an immense sense of loss."
Abdur Rashid Siddiqui, vice-chairman of the Islamic Foundation, UK, said: "This extraordinary leader of the Roman Catholic Church will be remembered for his unique and impeccable credentials as a champion of the cause of suffering peoples of the world."