Tens of thousands of pilgrims have flocked to one of Christianity's most popular shrines ahead of Pope Benedict XVI arrival in Fatima.

The Pope's personal pilgrimage to the shrine is being billed by the Church as the highlight of his visit.

On his arrival, the Pope is scheduled to immediately go to pray at the statue of Our Lady of the Rosary in the Chapel of Apparitions.

While the Pope will travel by helicopter, Church authorities say some 40,000 pilgrims are expected to arrive in Fatima by foot from across Portugal.

Many will cover the last few hundred metres of their journey on their knees in gratitude for favours they believe were granted to them by the Virgin.

'I have done this pilgrimage for 22 years because my daughter was at the doors of death and Our Lady granted me the miracle of saving her,' said Maria de Fatima, after travelling some 220km to Fatima.

Yesterday, while flying to an open-air mass in the capital, Lisbon, he said that the Church's 'truly terrifying' problems come from sin within the institution.

His comments are being billed as his strongest yet on sexual abuse by priests.

'Today, we see in a truly terrifying way that the greatest persecution of the Church does not come from outside enemies, but from the sin within the Church itself,' the Pope said while on the plane.

In an editorial, the Portuguese daily newspaper, Publico, said of the comments: 'With this Benedict XVI wanted to 'resolve' right at the start of his visit a controversy that could overshadow the trip and at the same time make it clear before believers and non-believers that his position is not to tolerate inadmissable practices,'

Experienced Vatican watchers see the comments as an important shift in papal tactics.

High profile 'Vaticanista' Marco Politi said the pope's comments were 'above all an internal message' aimed at members of the clergy 'who are resisting the pope's volition for zero tolerance' and are practising 'a form of silent sabotage'.

The Vatican has previously blamed the media and the Church's opponents for escalating the scandal. Cardinals accused the media of anti-Catholic 'propaganda' and the Pope's personal preacher compared the attacks on the Church as similar to anti-semitism.

The 83-year-old pontiff told the 80,000 attending the open-air mass in Lisbon yesterday that nothing could destroy the Catholic Church.

'The resurrection of Christ assures us that no adverse power will ever be able to destroy the Church,' he said during his homily on the banks of the Tagus river.

He said the victims' need for justice had to take priority. The Church had a 'profound need' to 'learn forgiveness and also the necessity of justice,' but he underlined that 'forgiveness does not replace justice'.

A US victims' support group dismissed the comments as meaningless and a poor substitute for action.

'There is no action,' said Barbara Blaine, founder and president of the Survivors Network of those Abused by Priests.

'Previous to this, he was pointing fingers at the press, saying you were anti-Catholic. He said victims who spoke up were engaging in petty gossip.

'He may have stopped that extreme pointing of fingers, but he hasn't taken any action that will protect children,' Ms Blaine told AFP reporters.

The papacy has been rocked by allegations that the Vatican protected paedophile priests from prosecution in Europe and the US, and that the Pope knew about several cases of predator priests prior to becoming Pope, but took no action against them.