Pope Francis today celebrated a huge open-air Mass in the centre of Seoul, where he denounced the growing gap between the haves and the have-nots, urging people in affluent societies to listen to "the cry of the poor" among them.

The Pope made his remarks in the homily of a Mass where he beatified 124 Korean martyrs who were killed for refusing to renounce Christianity in the 18th and 19th centuries.

Beatification is the last step before sainthood in the Roman Catholic Church.

In his homily before a crowd estimated at as much as one million Pope Francis said the martyrs' courage and charity and their rejection of the rigid social structures of their day should be an inspiration for people today.

The Pope said the Mass from a white altar platform in front of Gwanghwamum Gate, where some of those beatified by Pope Francis were killed during the Chosun dynasty.

The history of Christianity in Korea is unique in that it was not founded by Western missionaries. Korean intellectuals in the late 18th century heard about it through literature that had arrived in the country from China and developed their own community.