Pope Francis has beatified 124 South Korean martyrs, who were killed in the 18th and 19th centuries for refusing to renounce Christianity.
Beatification is the last step before sainthood in the Roman Catholic Church.
In the homily of a huge open-air Mass in the centre of Seoul today, he urged people in affluent societies to listen to "the cry of the poor" among them.
Before a crowd of hundreds of thousands, 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.
It was a theme the pope has been repeating since he arrived in South Korea on Thursday for his first trip to Asia since his election in March 2013, and has been a lynchpin of his papacy.
Last year, in the first major written work of his papacy, Francis attacked unfettered capitalism as "a new tyranny", urging global leaders to fight poverty and growing inequality.
Rapid economic growth has made South Korea one of the world's wealthiest countries, but it has also become increasingly unequal, with nearly half the elderly in poverty.
The pope said the Mass from a white altar platform in front of Gwanghwamun Gate, where some of those beatified by Francis were killed during the Chosun dynasty.
The pope also prayed with family members of victims of this year's Sewol ferry disaster.
One handed Francis a letter and said: "Please do not forget."
The Sewol ferry sank during a routine voyage on 16 April, killing more than 300 people, most of them children on a school outing.
The tragedy left the country grieving and outraged and prompted a backlash against the government of President Park Geun-hye over its handling of the disaster.