Pope John Paul II, in one of the most important religious events in Britain since it turned away from Catholicism in the 16th century, joined leaders of non-Catholic churches in an extraordinary pilgrimage to Canterbury today to promote Christian unity.

Nearly 450 years after Henry VIII broke with Rome, the pope and the archbishop of Canterbury, Robert Runcie, leader of the Church of England and the now worldwide Anglican Communion, worshiped together for the first time in what the pope called "this historic day, which centuries and generations have awaited."

They clasped hands, embraced and intermingled their voices in prayers and blessings with a congregation that included leading clergy of Britain's Catholic, Anglican and "free" Protestant churches. They knelt and prayed at the spot where Archbishop Thomas Becket was slain in 1170 for defending the medieval Catholic Church against Henry II.

They joined with the Methodist moderator of Britain's Free Church Federal Council to lead the congregation in a renewal of baptism ritual that the pope said "we share in common, Anglicans and Catholics." Ending this unprecedented ecumenical gesture, the three church leaders pronounced in unison, "This is the faith of the church."

In one of many references to the goal of Christian unity that filled the service, the pope said, "I appeal to you in this holy place, all my fellow Christians, and especially the members of the Church of England and the members of the Anglican Communion throughout the world, to accept the commitment to which Archbishop Runcie and I pledge ourselves anew before you today." The Episcopal Church in the United States is part of that Anglican Communion.

Canterbury Cathedral itself, a Christian shrine since before the time of Chaucer, "is an eloquent witness," the pope said, "both to our long years of common inheritance and to the sad years of division that followed."

Runcie noted that English Christianity was revived in Canterbury when pope Gregory the Great sent Augustine as a missionary to the Anglo-Saxons in 597. After building the predecessor of the 850-year-old cathedral standing today, Augustine became the first leader of the medieval church in England as archbishop of Canterbury.

"I rejoice that the successors of Gregory and Augustine stand here today in the church which is built on their partnership in the gospel," Runcie said. "If we can lift our eyes beyond the historic quarrels which have tragically disfigured Christ's church and wasted so much Christian energy, then we shall indeed enter a faith worthy of celebration, because it is able to remake our world."

The emotional service ended with the pope embracing Anglican bishops from throughout Britain and the world, and the other British church leaders. One of many outbursts of spontaneous applause from the congregation, dominated by Anglican clergy and lay leaders, greeted the pope's warm embrace of former archbishop of Canterbury Michael Ramsey, who began the developing Anglican-Catholic dialogue of unity by agreeing on a "common declaration" of purpose with pope Paul VI in 1966.

The pope and Archbishop Runcie signed another "common declaration" today calling for renewed efforts to resolve difficult doctrinal differences still remaining after 14 years of negotiations as "the next stage of our common pilgrimage in faith and hope towards the unity for which we long."

They decided to set up a new Anglican-Catholic international commission to tackle such stubborn problems as the Roman Catholic doctrine of papal infallibility and dogma about the Virgin Mary that have prevented Anglicans and Catholics from formalizing a closer relationship, or even taking communion together. "We commit ourselves anew," the pope and archbishop pledged, "to the task of working for unity with firm faith, renewed hope and ever deeper love."

Afterward, the pope talked for 90 minutes over a buffet lunch in the cathedral dean's home on a sun-drenched medieval courtyard with leaders of most of Britain's other non-Catholic denominations, including the churches of Scotland and Wales, the United Reformed Church, the Greek Orthodox Church, the New Testament Church of God, and Methodists, Baptists and Quakers.

One participant, Bishop Alastair Haggart, head of the Scottish Episcopal Church, said he had spoken to the pope about the problems of marriages between Catholics and non-Catholics, segregated schools and the inability of worshipers of different denominations to take communion together.

Opponents of the pope's visit sponsored a demonstration in London, however, with about 1,000 Protestants holding a service at Trafalgar Square to demand a halt to the Christian unity movement. The 170-foot column below the statue of Adm. Horatio Nelson was draped with a banner reading "Time to Repent."

The Rev. David N. Samuel, general secretary of the Protestant Reformation Society, called on Britain to break diplomatic ties with the Vatican, which were restored last March.

"We feel that some sort of counteraction is very necessary now to restore the true doctrinaire basis of the Church of England . . . which is being eroded," Samuel said.

In one of the most poignant moments of the service in Canterbury Cathedral, church leaders lit candles in a new chapel dedicated to 20th century martyrs. The pope lit a candle for a Polish priest who once was his tutor, Maximilian Kolbe, who later as a prisoner in Auschwitz volunteered to die in place of a man with a family. He is to be canonized a saint this year.

Archbishop Runcie honored San Salvador's Roman Catholic archbishop Oscar Romero, who was murdered in March 1980. Archbishop Timothy Olufosoye of Nigeria lit a candle for the archbishop of Uganda, Janani Luwum, who was tortured and killed during the regime of Idi Amin. A black Pentecostalist from Brixton in London, Esme Beswick, lit a candle for the Rev. Martin Luther King Jr., who was assassinated in 1968.

The pope said he was encouraged to appeal vigorously for renewed efforts to achieve Christian unity "by the witness of so many who have professed their faith in Jesus Christ through the centuries, often at the cost of their own lives, a sacrifice which even today is asked of not a few, as the new chapel reminds us."

Later today, the pope flew by helicopter to London's Wembley Stadium to say mass for an overflow crowd of more than 100,000 people, many of whom could only listen to loudspeakers in parking lots until the pope rode outside to see them.

This first truly public mass of his six-day tour of nine British cities produced familiar scenes of demonstrative adulation for the pope. His homily, devoted primarily to extolling Catholic traditions, was frequently interrupted by applause and cheers. The pope added an extra, unwritten prayer to the service for the dead and wounded on both sides in the conflict between Britain and Argentina over the Falkland Islands.