VATICAN CITY (Reuters) - Pope John Paul turns 80 on Thursday, showing the signs of ailment and advancing age yet retaining the hallmark vigor of his 21 years on the papal throne as leader of the world's one billion Roman Catholics.

The Pope normally lets his birthday pass with no celebration. This year is different.

The Pope will lead mass with 3,000 priests, 250 bishops and scores of cardinals in St Peter's Square at 10 a.m. In the evening, the London Symphony Orchestra will play Joseph Haydn's ''The Creation'' in a concert in his honor.

And on Friday, the Pope will address ambassadors from the 173 countries and institutions that have diplomatic relations with the Holy See.

The Polish Pontiff, the first non-Italian to head the See of St Peter in over 450 years, has seen and touched more people, met more leaders, and visited more countries than any before him.

The pace he set himself when, as Archbishop of Krakow, he was elected pope in October 1978 has been unrelenting.

His 92 journeys to date outside Italy to 123 countries and territories earned him the sobriquet of globe trotting pontiff.

On his doctors' insistence, his more recent trips have meant fewer encounters, but he still fits in a schedule which leaves aides and followers many years his junior out of breath.

Now stooped when he walks, shuffling sometimes due to a hip operation and troubled by what many believe to be Parkinson's Disease, the Pope still believes that he, as representative of Christ on earth, has to be seen by the faithful.

His desire to visit communist China has not waned and, if differences with the Russian Orthodox Church can be eased, he also wants to travel to Moscow.

This year alone he has looked at Mount Sinai in Egypt, fulfilled his dream to follow in Christ's footsteps in the Holy Land and revisited Fatima and its shrine to the Virgin Mary, who he believes interceded to save his life when he was shot in an assassination attempt in St Peter's Square in 1981.

He declared 2000 a Holy Year and the list of events at which he has presided and has still to preside has him in the public eye nearly every day of the Jubilee Year.

Birthday Tributes

The Pope is clearly seen not just as a religious leader but also as a respected global figure who stepped on to the world stage when Leonid Brezhnev was in the Kremlin, Jimmy Carter in the White House and Margaret Thatcher had yet to enter Downing Street.

Tributes from Catholic and non-Catholic leaders alike have begun pouring in.

``Pope John Paul...is an exceptional man,'' Mikhail Gorbachev, the last leader of the atheist Soviet Union, said in a birthday message in Thursday's Vatican newspaper Osservatore Romano.

``The most important thing is that the Pope did everything possible to help humanity out of its era of hate,'' he added.

The Pope was instrumental in the fall of communism in Eastern Europe through his support of the Solidarity movement in his Polish homeland.

``Europe owes you much,'' Italian President Carlo Azeglio Ciampi said in his message to the Pope. ``The world owes you much for your tenacious commitment to the defense of man.''

His hardline conservatism over dogma and morals disillusioned many liberal Catholics who had hoped his papacy would see some move toward an easing in the teaching on birth control, women clerics, homosexuality and marriage for priests.

Yet even the most lapsed of Catholics cannot deny the Pope has used his moral authority to the good to stare down dictators in his fight for human rights and dignity.

Israeli Prime Minister Ehud Barak said of the trip to the Holy Land by the Pope, who as a boy played soccer with his Jewish schoolmates before the Holocaust: ``Your sensitivity, your gestures conquered the hearts of all Israelis.''

And from the Muslim Palestinian leader Yasser Arafat: ``We pray God give you a long life, good health, serenity and peace so that you can continue to spread the Christian message of love and peace.''