Pope Francis has sent a special video message to the people of Portugal as he prepares to travel to Fatima to canonise on Saturday two of the three children who saw six visions of a woman dressed in white, believed to have been the Virgin Mary.

The first vision was in May, 1917.

The two children being canonised, Jacinta and Francisco Marto, are the two who died young, in 1919 and 1920, just a couple of years after their last vision. The third child, their cousin Lucia, became a nun and died in 2005, aged 97.

In a video message, released today, Pope Francis says: 'Just hours from my pilgrimage to Our Lady of Fatima, I find myself in a state of joyful expectation for our upcoming encounter at the house of the Mother.'

He says he is sorry he could not respond to the many invitations to visit people in their homes.

He adds: 'I thank you for the understanding with which my decision to keep my visit circumscribed to the Fatima Sanctuary, where I hope to meet you at the feet of the Virgin Mother, has been received.

'It is in my role as universal pastor that I am about to present myself to her and I need to feel you close, physically or spiritually so that we are one heart and one mind.'

He has adopted as his logo for the visit: 'With Mary, I come as pilgrim in hope and in peace.'

He describes himself in the video as 'a sinner amongst sinners'.

Up to a million people are expected to attend the canonisation on Saturday at the shrine for the children, who received messages from the woman in white that came to be known as the three secrets of Fatima.

More than eight in 10 of Portugese are Catholic. 

The three secrets, eventually written down by the third child who became a nun, were said to be about Hell, the two World Wars and the attempted assassination of Pope John Paul II.

Father Vitor Coutinho, deputy head of the sanctuary of Fatima, said the Pope was showing the importance of Fatima to the church by carrying out the canonisation there rather than in Rome, where they are usually done.

'Therefore he is confirming that a life lived in the light of the spirituality of Fatima can lead to sainthood,' he said.

Huge concrete blocks have been erected around the sanctuary to protect it and the crowds from possible terror attack.

Additional reporting by Reuters