Polish pilgrims are travelling to Rome by train to witness Pope John Paul II become a saint. (Credit: Reuters) 

As many as one million people are expected in Saint Peter's Square and nearby streets of Rome to witness the canonisation of Popes John XXIII and John Paul II later today.

John XXIII, who reigned from 1958 to 1963 and set up the modernising Second Vatican Council, and John Paul II, who reigned for nearly 27 years and played a leading role on the world stage, will be declared saints by Pope Francis.

Francis' own huge popularity has added extra appeal to the unprecedented ceremony to raise the two former leaders of the Church to sainthood.

But while both were widely revered, there has also been criticism that John Paul II, who only died nine years ago, has been canonised too quickly.