By Philip Pullella and Ju-min Park

KKOTTONGNAE South Korea (Reuters) - Pope Francis on Saturday issued a clear warning to Roman Catholic clergy, saying those who profess poverty while living rich material lives were hypocrites who hurt the image and mission of the Church.

On the third day of his visit to South Korea, Francis celebrated a huge open-air Mass in the center of the capital Seoul, where he denounced the growing gap between the haves and have nots, urging people in affluent societies to listen to "the cry of the poor" among them.

Later, he flew by helicopter to a hilltop center for the sick, disabled and homeless run by the Church in the town of Kkottongnae, southeast of Seoul.

There, he comforted sick children and adults, some of them severely disabled and disfigured and in wheel chairs, and declined to use a comfortable white, padded chair that had been prepared for him. "I like to stand," he said. Bowing to local tradition, he removed his shoes as he entered the center.

Later, in another section of the institute, Francis praised clergy who dedicate their lives to the needy and urged them to stay on the right path.

"The hypocrisy of those consecrated men and women who profess vows of poverty, yet live like the rich, wounds the souls of the faithful and harms the Church," he said.

Francis has been urging Roman Catholic officials to live simpler lives, and renounced the papal apartments in the Vatican palace for modest quarters in a Church guest house.

In March, he removed a German prelate who became known as the "bishop of bling" because he spent 31 million euros ($41.5 million) of Church funds on an extravagant residence.

In the United States, the Archbishop of Atlanta apologized for building a $2.2 million mansion to use as his home, a move that made him the object of derision and complaint, and said it would be sold.

On his first day in South Korea on Thursday, Francis made a splash with his choice of car for the five-day visit, a modest locally-made Kia Soul.

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At the hilltop center, he joked with nuns that he had to cut short his time with them because if it went beyond dark "the helicopters risk crashing into the mountain".

Earlier on Saturday in Seoul, the pope 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.

HEAR 'CRY OF THE POOR'

In his homily before a crowd of hundreds of thousands in Seoul, 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.

"Their example has much to say to us who live in societies where, alongside immense wealth, dire poverty is silently growing; where the cry of the poor is seldom heeded and where Christ continues to call out to us, asking us to love and serve him by tending to our brothers and sisters in need," he said.

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 the papacy of the first non-European pontiff in 1,300 years.

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.

During his procession to the altar, Francis stopped to pray with family members of victims of the Sewol ferry disaster, one of whom handed him a letter and said: "please do not forget." The Sewol capsized and sank during a routine voyage on April 16, killing more than 300 people, most of them school children.

As he did on Friday when he prayed for the victims, survivors and families of the disaster, the pope wore a yellow ribbon, the symbol of tribute for the ferry victims.

In Kkottongnae, Francis watched a group of disabled children perform a dance, and a woman paralyzed from the waist down gave the pope an embroidery with his image on it.

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.

The Catholic Church has been growing rapidly in South Korea, doubling in the past 25 years to account for about 11 percent of the population of 50 million. About 100,000 Catholics are added every year.

(Additional reporting by Kahyun Yang; Editing by Tony Munroe and Richard Borsuk)