Pope John Paul II began his historic, almost postponed visit to Britain today by appealing repeatedly for an end to the war over the Falkland Islands and calling for Christian unity in a troubled time in world history.

"My visit is taking place at a time of tension and anxiety, a time when the attention of the world has been focused on the delicate situation of the conflict in the South Atlantic," the pope said in a long and somber address after landing at Gatwick Airport south of London this morning to begin his 12th foreign tour.

Crowds numbering tens of thousands in places--and including scattered hard-line Protestant demonstrators--lined routes of John Paul's travels as he opened the first papal visit in history to Britain.

The trip, which Vatican Radio today called the most important ecclesiastical event for Britain since Henry VIII's break with Rome in 1534, is intended to be a major step toward mending the 4 1/2-century rupture between the Roman and English churches.

Today, however, the Falklands war appeared uppermost in John Paul's mind. Referring to his recent formal proposal to Britain and Argentina for an immediate cease-fire and restart of negotiations, which was rejected by British Prime Minister Margaret Thatcher, he said, "I have sought to encourage a solution which would avoid violence and bloodshed. As I stand here today, I renew my heartfelt appeal and I pray that such a settlement of the dispute will soon be reached."

Later, in his homily at a nationally televised mass at Westminster Cathedral midway between the houses of Parliament and Buckingham Palace in central London, the pope served notice that he will make his plea in each of the nine cities he is to visit during his six days in England, Scotland and Wales.

"As we proceed to celebrate the mystery of our faith, we cannot forget that an armed conflict is taking place. Brothers in Christ are fighting a war that imperils peace in the world," he told a congregation of British Catholic bishops, clergy and lay leaders, representatives of other British churches and members of Parliament.

"I ask you to join me at each step of my pastoral visit in praying for a peaceful solution to the conflict," he implored, "praying that the God of peace will move men's hearts to put aside the weapons of death and to pursue the path of fraternal dialogue."

Later, after moving through demonstrative crowds of tens of thousands on both banks of the Thames River, the pope departed from his text at a service for 4,000 invalids and terminally ill people at St. George's Cathedral in the rundown area of Southwark to say, "As we speak of suffering, affliction and death, we cannot forget those who have suffered and died during the armed conflict in the South Atlantic."

And again, at supper tonight with the Catholic bishops of Britain, the pope urged them to pray with their "Argentine brothers" for peace and to publicly "proclaim that peace is possible."

The visit was nearly canceled because of the Falklands war. Despite two years of preparations by Britain's 5 million Catholics, less than 10 percent of the total population, it was not certain the pope would be coming until earlier this week when it was announced he also would visit Argentina next month.

Cardinal Basil Hume, Roman Catholic primate of England and Wales and one of several British archbishops who successfully pleaded with the pope to come here despite the war, said in his welcoming speech, "We appreciate your presence among us even more deeply because of the many difficulties and uncertainties which have beset preparations for your visit."

To make the visit less objectionable for Latin American Catholics, the British government was not represented at today's ceremonies and Thatcher is not scheduled to meet with the pope. But John Paul did visit Queen Elizabeth II, in her role as constitutional head of the Church of England, this afternoon at Buckingham Palace. Returning a visit she made to him at the Vatican 18 months ago, he spoke alone with her for half an hour in a room overlooking the palace's vast gardens.

"God bless your son," the pope was heard to say to the queen after their meeting. Her second son, Prince Andrew, 22, is a helicopter pilot with the British task force in the Falklands.

At Westminster Cathedral, the pope cited today's world "scarred by hatred and injustice and divided by violence and oppression" as evidence of a need for Christian unity and reconciliation. Saying that "we should be completely one" after the the long estrangement of Christians here, he told Britons that "my deep desire, my ardent hope and prayer, is that my visit may serve the cause of Christian unity."

Before they were overshadowed by the Falklands crisis, the primary purposes of John Paul's visit, according to church leaders here, was to raise spirits in Britain's relatively small Catholic Church and to encourage closer relations between it and the dominant Church of England, created by Henry VIII's break with Rome and made Britain's state church by his daughter, Elizabeth I.

The ecumenical centerpiece of the visit is to be a pilgrimage by the pope with the archbishop of Canterbury Saturday to the focal point of Anglicanism, Canterbury Cathedral, where they will participate in an ecumenical service and meet with the leaders of most of Britain's Protestant churches.

Militant Protestants who object to the pope's visit here have planned demonstrations for many places on his itinerary. Fourteen militant antipapist Protestants from Northern Ireland, including six clergymen from the Rev. Ian Paisley's Free Presbyterian Church in Belfast, were arrested after a brief demonstration at Victoria Station where the pope arrived on a train from Gatwick.

As the pontiff passed in a white Land Rover converted into a glassed-in "popemobile," the demonstrators waved placards saying, "Jesus saves, Rome enslaves" and "Pope John Paul, anti-Christ."

But they were the exceptions among otherwise enthusiastic people, including thousands of schoolchildren, who lined the route of the pope's travels. Though considerably smaller than the crowds attracted by the pope in predominantly Catholic Ireland three years ago, they were no less enthusiastic. Many non-Catholics were among those who spent a night sleeping on sidewalks to have the best views of the pope today.

"He kissed me and said, 'God bless you,' " said 10-year-old Sharon McArrick from London. "It was wonderful. He was so nice."

Henry VIII broke with Rome just 12 years after receiving from the pope the title "defender of the faith," which the British monarch retains to this day, for publishing a defense of Catholic doctrine against Martin Luther's attacks. But the pope would not dissolve Henry's marriage to Catherine of Aragon to legitimize his union with Anne Boleyn. In 1559 Elizabeth I, daughter of Henry and Anne Boleyn, made the Church of England and the state a single entity and in 1570 she was excommunicated by Pope Pius V.

For several centuries, Catholics were unable to vote or hold public office, meet for worship or inherit land in Britain. Under persistent persecution, they dwindled to only about 70,000 in a population of 8 million by 1780.

But anti-Catholic laws were abolished in 1829.