The Vatican celebrated the 50th anniversary on Thursday of a Council that changed the face of Catholicism, as it tries to rekindle the religious fervour of the time amid rising secularism.

Hundreds of bishops from around the world held a solemn procession through St Peter's Square, after which Pope Benedict XVI celebrated an open-air mass to announce a new global "Year of Faith" for the first time in 45 years.

"If today the Church proposes a new Year of Faith and a new evangelization, it is not to honour an anniversary, but because there is more need of it, even more than there was 50 years ago!," the pope told gathered crowds.

"Recent decades have seen the advance of a spiritual 'desertification'. We see it every day around us. This void has spread," he added.

The day will close with a torchlit procession along the main avenue leading to the steps of St Peter's Basilica and an unusual evening greeting from the pope from his apartment window, echoing those dramatic days of 1962.

The Archbishop of Canterbury Rowan Williams, the spiritual leader of the Anglican Church, and the Ecumenical Patriarch Bartholomew I of Constantinople also attended along with 14 of the original "fathers" from the 1962 Council.

The Beatles had just released their first single and James Bond had just hit the silver screen when 2,250 Catholic bishops from 116 countries met in Rome in 1962 to reform rituals and beliefs that appeared increasingly anachronistic.

Pope John XXII hailed the crowds of faithful on the night of October 11, 1962 with a famous heartfelt speech in which he called on believers to pursue "faith, hope, charity, love of God, love of brothers and the common good."

The Second Vatican Council -- dubbed Vatican II -- only wrapped up in 1965 after adopting key reforms including the chance to celebrate mass in other languages than Latin and the idea of opening dialogue with other faiths.

Benedict was then Joseph Ratzinger, a 35-year-old German priest and among the most reformist voices at the Council. He has defended the achievements of Vatican II against traditionalist voices in the Church that still question it.

Reformists in the Church who campaign in favour of the ordination of women and allowing priests to marry, however, accuse the pope of becoming increasingly conservative and turning his back on the ideals of Vatican II.

In a personal remembrance published in the Vatican's official daily L'Osservatore Romano the pope remembered October 11 as "a splendid day".

"There was a general feeling of expectation in the air," he said.

The pope also dismissed as "absurd" the claim that Vatican II marked a rupture with tradition and said it was "a positive step for changing times."

The "Year of Faith" lasts until November 24, 2013 and is intended "to help believers in Christ to have a more informed and reinvigorated adherence to the Bible, especially at a time of deep change that humanity is experiencing".

On Thursday, the pope described lack of faith as a desert-like void but said people should use the "experience of this desert" to re-kindle their belief.

"This is how we can picture the Year of Faith: a pilgrimage in the deserts of today?s world," he said.

The 85-year-old pontiff also highlighted the achievements of international personalities, such as pious Catholic Urzua Iribarren, one of the 33 Chilean miners famously rescued from underground two years ago.

Among the other people to be recognised is Fabiola Gianotti, an Italian physicist in charge of the ATLAS programme at the Large Hadron Collider at CERN in Switzerland, who announced the discovery of the "God Particle" this year.

At a closed-door meeting with the pope earlier this week, world bishops vented some of their frustration with the current state of the Catholic Church.

"We have lost credibility," Italian Archbishop Salvatore Fisichella, who heads up the pontifical council for evangelisation, said at the meeting.

"We have turned a life of faith and ritual into bureaucracy," he said.

One European archbishop said: "Evangelisation has been hurt and continues to be impeded by the arrogance of its messengers. The hierarchy must shun arrogance, hypocrisy and bigotry.

"We must punish the errant among us instead of covering up our own mistakes," he said, in an apparent reference to the child sex-abuse scandals in the Church, which involved bishops accused of protecting suspects.