Pope Benedict has begged forgiveness from God and victims of child sexual abuse by priests.
He has also promised that the Catholic Church would do everything in its power to ensure that it never happens again.
Pope Benedict said the year that was to have celebrated the priesthood had been marred because 'the sins of priests came to light, particularly the abuse of the little ones.'
The comments from Benedict are some of his clearest ever about the scandal that has swept the Church around the world.
He was speaking to some 15,000 priests at a homily in St Peter's Square to conclude the Roman Catholic Church's 'Year of the Priest' celebrations.
He said: 'We too insistently beg forgiveness from God and from the persons involved, while promising to do everything possible to ensure that such abuse will never occur again.'
The 83-year-old pope also promised that the Church would enact stronger controls on choosing men who enter the priesthood.
'In admitting men to priestly ministry and in their formation we will do everything we can to weigh the authenticity of their vocation and make every effort to accompany priests along their journey, so that the Lord will protect them and watch over them in troubled situations and amid life's dangers,' he said.
He said the worldwide community of Catholic priests, numbering more than 400,000, should see the sexual abuse scandal and its repercussions as 'a summons to purification' for themselves and for the entire Church.
The Church has seen fresh revelations of past sexual abuse by priests in a number of places, particularly in Europe and the US.
The pope himself has come under scrutiny over a case in which a paedophile priest who was to have undergone therapy was instead allowed to resume duties when the pope was archbishop of Munich in his native Germany more than 30 years ago.
The Church said a subordinate was responsible for the decision.
Five bishops in Europe have already resigned.
One has admitted sexual abuse, another is under investigation and three have stepped down over their handling of abuse cases.
The abuse scandal has hit Catholic communities in Ireland, Belgium, Germany and the US particularly hard.
A poll two months ago in Germany showed that a majority of people had lost confidence in the Church and about a quarter of the country's Catholics are considering quitting.