In white cassock and rich red cape, Pope John Paul II emerged from an Aer Lingus Boeing 747 at 3:04 this afternoon, knelt to kiss the tar and stone runway at his feet, and began the first U.S. papal tour in history, a seven-day journey that this traveling pontiff called "a pastoral visit."

There to greet him at Logan International Airport were First Lady Rosalynn Carter, a seemingly endless line of political dignitaries and church leaders, and a dreary gray mist that blew in from the nearby harbor, all of which the pope embraced in irrepressible spirit.

"I come as one who already knows you and loves you," he said, his words flowing out in accented but articulate English. "Once again I can now admire firsthand the beauty of this vast land stretching between two oceans."

The rain-slicked runway, the leaden skies and the grueling day that for him began at dawn in Ireland could not keep the pope from exuding warmth and love during his first greetings as pope on American soil. In strong and joyous voice, he responded to the First Lady's welcome with the words, "Praised be Jesus Christ." Then, a few minutes later, he ended his arrival speech by reciting the final lines of the song "America the Beautiful."

After the brief airport ceremony, witnessed by some 400 invited guests and an equal number of journalists, the pope was ushered to an open-top car that transported him through the streets of Boston. He stood and waved as the motorcade moved grandly past crowds that in places lined 10 to 12 deep on the curbs and sidewalks of this heavily Catholic city.

At each sighting of the pope standing high in his black limousine, his vestments soaked with water, echoes of excitement reverberated through the streets. The pope had said he loved America, and the people of Boston answered in kind, standing in the drizzle for hours awaiting his words, to weep, to scream and to sing.

Waiting for him at Boston Common was a crowd estimated by police at about 250,000. Not even the persistent rain could dampen the spirits of this crowd. For several minutes before the pope moved onto the stage, the quarter-million people seemed as one as they sang "The Battle Hymn of the Republic" -- verse after verse, louder and louder.

As the pope strolled out from the parking garage under the common on his way to the stage, ripples of applause rolled across the park in ever stronger increments until finally the scene was one sustained roar. Then, wearing a red and gold mitre, the pope issued a sharp homily aimed at the youth of this country.

"Faced with problems and disappointments, many people will try to escape from their responsibility," said the pope, who has frequently deplored the pleasures which lure youth away from traditional moral values. He said they try to "escape in selfishness, escape in sexual pleasure, escape in drugs, escape in violence, escape in indifference and cynical attitudes.

"But today, I propose to you the option of love, which is the opposite of escape."

The pope said that young people should turn to the priesthood or be of special service to society, "especially to the needy, the poor, the lonely, the abandoned, those whose rights have been abandoned, those whose rights have been trampled upon, or those whose basic needs have not been provided for."

Everywhere he has traveled during his first year as pope, John Paul II has shown a particular interest in young people. In Mexico, in Poland, in Ireland and tonight in Boston, he told them that "the day of tomorrow belongs to you."

"Again and again, I find in young people the joy and enthusiasm of life, a searching for truth and the deeper meaning of the existence that unfolds before them in all its attraction and potential," he said.

"Do I then make a mistake when I tell you, Catholic youth, that it is part of your task in the world and the church to reveal the true meaning of life when hatred, neglect or selfishness threaten to take over the world?"

Thousands of the people who came to the Common were too far away to hear the pope's message. They jumped in the air to catch a glimpse of him and cheered when those up close, who could see and hear, cheered. But for most of the people of Boston, the most stirring event of the day was the papal motorcade through the city.

On the roofs of brittle frame houses in East Boston, vantages from which one could see the pope for a few seconds, people cheered and wept. Babies were held high in the air to be seen by the pope. Gold and white pennants-- the papal colors-- churned the air. Across one building was strung a banner that read: "Eastie Loves the Pope." On another were written words of welcome in Polish.

The motorcade sped through the Sumner Tunnel and emerged to the equally exuberant crowds of the Italian North End. There, hundreds of children sprinted through alleyways and across playing fields to catch two and three sightings of the smiling pontiff. From restaurants hung banners proclaiming "Viva il Papa."

At every corner, school and church bands had taken up positions so that the pope's coming could be heralded by hundreds of bugle-blowing children in colorful uniforms.

The pope saw some of the grimmest sections of the city. Parts of Roxbury looked like they had been abandoned, with vacant lots full of rubble and weeds everywhere the pope looked. But even there people had suddenly reappeared to cheer him on. The motorcade moved on past the vast complex of Boston City Hospital where white-jacketed doctors stood on barrels to see him and the old and infirm smiled weakly or gingerly raised their hands from their wheel chairs to wave as he passed.

When he entered the Cathedral of the Holy Cross in the South End, 2,000 priests and deacons stood on top of the pews screaming, clapping and chanting until the high vaulted gothic colums seemed to vibrate with the noise. The pope prayed silently before the altar of St. Joseph, and the cathedral, took, fell silent for a few seconds. But the moment he arose, the thunder began again. He allowed it to continue as he sat on the papal throne listening to the choir, his fingers to his lips in thought, occasionally glancing at a small contingent of press photographers who continued to click their shutters and scramble on top of each other even after he had gestured to the priests to quiet them, as if to say, "enough."

After a lengthy introduction by Cardinal Humberto Medeiros, the pope rose and, perhaps in jest, addressed the crowd as "Dear brothers and several sisters." There were no women in the crowd, except for a nun taking pictures for the Boston Pilot. The priests appreciated the humor.

The motorcade moved away from the cathedral along Massachusetts Avenue, then had to slow considerably as it neared the Boston Common. Along Boylston Street, the windows of fashionable shops were filled with faces, and elbowing throngs surged 20 deep. Scores climbed out on narrow ledges to watch the pope's passing. The ledges were sometimes three and four stories above the street.

The crowd at Boston Common was massive, but not as large as some city officials had anticipated. And a few minutes after the pope arrived, hundreds of people withdrew from the muddy grounds to nearby bars and restaurants. Many who came early brought quilts and sleeping bags to sit on while they waited for the pope's arrival. As the rain turned the grassy slopes into a muddy swamp, most of the crowd stood and the quilts were trodden in the mud.

The park was filled with young people like the Higgenbothams: Tom, 18, Ricky, 11, and Neil, 13. They had come from Cape Cod and got to the common early in the morning. "You only get a chance like this once in a lifetime," said Tom, who hoisted Ricky on his shoulders for a better view. As exciting as the sight of the pope was, Ricky said the anticipation of his arrival was even more joyous. He said boys and girls were dancing with excitement for an hour before the pontiff appeared.

Even after the pope had finished and was whisked off for a well-deserved night's sleep at the cardinal's residence in Brighton, thousands stayed at the Common, soaked to the bone, as they received communion from bishops and priests who maneuvered through the sloppy field for another 30 minutes. The sense of euphoria was perhaps strongest then; those who stayed considered it the last act of a day of sacrifice and sharing.

But not everyone on the Common was an admirer of John Paul or the Catholic Church. Noni Beale, 24, waved a placard which read: "No More Overpopulation: Yes on Birth Control."

She was accosted by an angry middle-aged man. "I am going to ask you once and only once -- will you put that thing down?" he said to her.

Beale responded with a tight smile and held on to her standard while the man glared at her. When she made no move to lower the placard, he ripped apart the water-soaked cardboard, smashed it into the mud and disappeared into the crowd to murmurings of approval.

Another act of protest came when more than 1,100 predominantly black Bostonians marched along a long segment of the papal route today to protest the shooting last Friday of a black high school football player. Three white teen-agers, have been arrested in connection with the shooting, which opened old racial wounds in the city.

Organizers of the march sought, to have the pope make a statement about the shooting and the city's racial climate that would be similar to the statement he made in Ireland on the violence there.

Convening at a settlement house in the predominantly black South End of Boston, where homes are rapidly being bought and renovated by whites, the group marched to Holy Cross Cathedral.

The group was cordoned off by rows of National Guardsmen and Boston policemen. There were no clashes along the march route. However, their loud chant, "Stop the Racist Attacks," led the papal motorcade to slightly alter its route as it left the cathedral.

The pope, while at Holy Cross Cathedral, made what some interpreted as a reference to the city's racial troubles.

"I am greeting a community that through the many upheavals of history has always been able to change and yet remain true to itself," said the pope. "A community where people of all backgrounds, creeds and races and convictions have provided workable solutions to problems and have created a home where all people can be respected in their human dignity."

For the 59-year-old pope, this eventful day began long before the sun rose over Boston. It was 7:30 a.m. in Dublin -- and 2:30 on the East Coast of the United States -- when he was ferried by helicopter out to the town of Maynooth for an address at St. Patrick's College, Ireland's major seminary.

There, he sounded the first notes of a conservative theme that would dominate his final hours in Ireland.

First he urged the priests and nuns at Maynooth to wear the traditional dress of the church and reject the trend towards "taking God off the streets by adopting secular modes of dress and behavior." Later, at a mass at the Limerick race track, he spoke out strongly against divorce and abortion; and urged women to spurn the lure of non-religious careers and give full attention to the family.

After the six-hour flight across the Atlantic from Shannon to Boston, the pope, who had only 13 hours of rest during his 2 1/2 days in Ireland, seemed relaxed and rested. His first act in America uas to open his arms in a gesture of humility and warmth, a gesture he repeated throughout the day.

John Paul II, a vigorous man uho clearly enjoys mingling lith the people, will have many more opportunities to use that winning gesture during his tour of the United States. In a schedule that uould exhaust the most well-conditioned, the pope is prepared to deliver more than 60 speeches in six cities over the next seven days. Tomorrow, he embraces New York.