``Long live the pope!'' cheered well-wishers by the thousands when he swung through St. Peter's Square in his open-top white ``popemobile.''
In a moment of exuberance, the pontiff lifted both hands from the vehicle's support bar and almost lost his balance. He quickly grabbed the bar again.
Celebrations were capped with an evening with a concert in his honor by the London Philharmonic led by Gilbert Levine in a Vatican auditorium.
John Paul's public day began with an affectionate greeting from a top Vatican prelate on the steps of St. Peter's Basilica.
``We feel guided by a man of God who has won love and respect beyond any human barrier,'' said Cardinal Dario Castrillon Hoyos in the name of the more than 6,000 priests from around the world who accepted the pope's invitation to help celebrate the birthday Mass.
``The Lord prepared an athlete to tirelessly walk the paths of the world,'' said the Colombian cardinal, referring to a younger image of Karol Wojtyla, the robust Polish bishop and skilled skier and hiker who became pontiff at age 58.
Then, came fond words for today's pope, whose movement and speech are hampered by age and by symptoms associated with Parkinson's disease, such as tremors. The cardinal offered thanks for ``those white hairs, for the suffering that has made you even dearer in our eyes, for your physically tired steps so spiritually intrepid....''
John Paul added his own ``hymn of gratitude to the Father of life, who allows me today to celebrate the Eucharist with you with the exultance of the perennial youth of the Spirit on the occasion of my 80th birthday.''
Later in the Mass, while a chorus sang ``alleluia,'' John Paul bowed his head, at times raising his clasped hands to his face, in apparent prayer or meditation.
Red roses decorated the altar area where the pope gave communion to some of the faithful chosen from the crowd.
At one point, the pontiff, whose staunch support of Poland's solidarity labor movement is credited with helping bring about the demise of Soviet bloc communism, seemed to be acknowledging his role in history.
A dozen priests who are the pope's age or older, as well as Poles and others who help run the papal household, were invited to join the pope and cardinals for lunch.
Cardinal Bernardin Gantin of Benin made a toast wishing the pope ``not only joy, but peace, health, longevity.''
English-speaking cardinals sang ``Happy Birthday,'' and Poles burst into a round of ``Sto Lat'' (May you live 100 years).'' Participants said the pope joined in the Poles for a bit of song.
``It was very informal. It's always this way when the cardinals get together with the pope,'' said Belgian Cardinal Godfried Danneels as he left the luncheon. It was held in the Vatican residence where cardinals will stay when the time comes to vote for John Paul's successor.
There was lobster antipasto, lots of pasta, pastry-wrapped roast beef and a large, flat white cake. Flowers decorated the room but there were no candles on the cake, the cardinals said.
Priests, from parish pastors to hospital chaplains, came to the Mass from different continents.
``It was great to see the pope looking so well,'' said the Rev. Seamus O'Kane from Ireland.
Raymond Igbodi, one of 40 priests who came from Nigeria for the occasion said: ``I am happy to be a priest like him.''
John Paul prayed during the Mass that men who had left the priesthood would ``keep alive in themselves the commitment for Christian coherence.''
The pontiff made his third pilgrimage to Fatima last week to give thanks for surviving a 1981 assassination attempt in St. Peter's Square. After he placed at the shrine's statue of the Madonna a gold ring given him by his mentor, the late Polish Cardinal Stefan Wyszynski, newspapers speculated the gift meant John Paul is about to relinquish the papacy.
The Vatican immediately branded the speculation as ''150 percent wrong.''
Among birthday greetings from around the world was one from Italian President Carlo Azeglio Ciampi praising the pope's ``spiritual impetus and intellectual youthfulness.''