Pope John Paul II is marking 25 years as leader of the world's Catholics. He celebrated mass before tens of thousands worshippers in St. Peter's Square, telling the adoring throng God was encouraging him to continue his papacy, despite his bad health.

"God, while knowing my human fragility, encourages me to respond with faith ... and he invites me to assume the responsibilities that he himself has entrusted to me," the 83-year-old Pope said in his homily Thursday.

John Paul cast his thoughts back 25 years, recalling the day he was elected pontiff.

"How could I, humanly speaking, not tremble? How could such a huge responsibility not weigh on me?" he asked.

The Pope began the mass at about 6 p.m. local time, around the same hour he first appeared to the world after his surprising election in 1978 as the first non Italian pope in 455 years.

John Paul was joined by cardinals from around the world for the festivities as well as about 50,000 believers. Millions more watched on television.

"In a sense it was a farewell address because he knows his time is short. But, on the other hand ... it was also very upbeat and optimistic," CTV's Larry Stout said.

The crowd broke into applause when a weary looking John Paul was wheeled out to the altar in his throne-like chair, dressed in vestments and a jewel-encrusted bishop's hat, while a choir sang hymns.

The silver anniversary is a milestone matched by only three of the Pope's predecessors. But, John Paul's ill health cast a long shadow over the anniversary festivities.

"There's no question about his frail health. It's very frail," former U.S. Ambassador to the Vatican Thomas Melady told CTV News.

While he suffers the effects of Parkinson's disease and arthritis, John Paul continues to defy skeptics with a taxing schedule, despite his inability to walk or even stand. And even though he increasing difficulty speaking, the pontiff still holds public audiences.

John Paul has also made clear there are no plans for him to step down, while Vatican officials have stressed he has the mental clarity to lead.

Earlier Thursday, the Pope addressed cardinals gathered at the Vatican. Relying on an aide to read parts of his speech, John Paul told them he shared the "anxieties, the sufferings, the hopes and the joys" of their ministry.

It was 25 years ago that the College of Cardinals gathered to name a successor to Pope John Paul I, who died after only 34 days in the papacy.

On that day, plumes of white smoke poured out of the Sistine Chapel chimney at 6:18 p.m., the sign the cardinals had found the next pope. At 6:45 p.m., the news was announced from the balcony of St. Peter's Basilica, and at 7:15 John Paul II made his first appearance.

Since then, John Paul has had a remarkable tour, helping to bring down communism in eastern Europe by sparking a mostly peaceful revolution in his Polish homeland; seeking to heal divisions between Christians and Jews; working tirelessly for world peace and even surviving an assassination attempt.

John Paul is the most-traveled pope ever, visiting 129 different countries in 102 foreign trips. He made more saints than any of his predecessors over the past 500 years combined -- a total of 476.

On Sunday, as part of his anniversary celebrations, he will beatify Mother Teresa, the nun who devoted most of her life to caring for the sick and destitute.

John Paul also ushered the Vatican into the modern age, becoming a media superstar and drawing cheering crowds wherever he went, particularly among the young.

In July, 2002, the Pope brought more than 800,000 Catholic pilgrims to Downsview Park in Toronto for a three-hour mass concluding that year's World Youth Day.

"The Holy Father has a keen insight that often in the Church and in society the young are forgotten. He has a keen appreciation for them and a keen love for them which is something they've picked up on," Rev. Joseph De Viveiros told CTV News.

"It was quite clear in the remarkable display of young people who came to Toronto ... they came to see this frail old man because he loves them and he displays the love of God for them."

On Catholic Church doctrine, John Paul has toed a conservative line, particularly in matters concerning what he called the sanctity of human life: abortion, contraception and euthanasia are all banned in the Catholic Church under John Paul.

He ruled out letting priests marry or allowing women to be ordained, and recently endorsed a worldwide campaign to keep same-sex unions from receiving legal recognition.

Such hard-line positions alienated some Catholics and sparked criticism the Vatican was out of touch with 21st century society. In particular, opposition to condoms has been blamed by some for hurting the fight against AIDS.

For all his successes, John Paul also had his disappointments: the Catholic Church's sex scandal in the United States weighed heavily on him and he has never realized his dream of visiting Russia.

John Paul has one more milestone looming -- he will become the third longest-serving pope if he surpasses Pope Leo XIII's reign of 25 years and four months about a century ago.

The first pope, St. Peter, served as leader of the Catholic Church for at least 34 years and is considered the longest-serving pontiff.

With a report from the Associated Press