A group of traditionalist Catholic worshippers have revolted against Pope Leo XIV by consecrating four bishops.
The Society of St Pius X held the ceremony on Wednesday in the Swiss town of Écône despite an appeal from the Pontiff to not continue with the event.
In a letter to the group, Pope Leo urged them to "turn back" and to carefully consider the "schismatic act you are about to undertake".
Vatican officials had warned the consecration could lead to the immediate excommunication of the bishops.
The Society of St Pius X has been at loggerheads with the Catholic Church for over five decades, but the consecration - which was livestreamed on YouTube - marks a dramatic escalation in tensions.
The group first formed in November 1970 in protest of the Vatican's efforts to modernise the church and rejects the decisions of the Second Vatican Council - held in the 1960s - which allowed priests to hold services in languages other than Latin.
It also rejects efforts to mend divisions between Catholicism and other Christian faiths, and insists on the primacy of the Roman Catholic Church.
Massimo Faggioli, a Vatican expert and professor of theology at Trinity College, Dublin, told The Telegraph: "It's an earthquake for the Church - but not a surprise. It was predictable."
The ceremony marks the biggest challenge to Pope Leo's authority
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"Everyone knew they never had any intention of changing their views on the Second Vatican Council, which they regard as heresy. It solidifies a rupture that began decades ago."
And Stephen Bullivant, who teaches theology at St. Mary’s University, told The New York Times the event was a proxy for the "polarised debate within the church around the Second Vatican Council, the current direction of the Catholic Church, all the troubles of secularisation and decline of priests".
The society had previously consecrated four bishops in 1988 - who were quickly excommunicated by Pope John Paul II.
The Vatican has not yet commented on the ceremony, but it marks the biggest challenge to Chicago-born Pope Leo's authority.
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The Society of Saint Pius X consecrate four bishops (pictured), who were threatened with immediate excommunication
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Mr Faggioli said the event was likely to make conservative Catholics "more critical of Pope Leo".
Revered Davide Pagliarani, the superior general of the society, admitted the consecration was an outward act of rebellion.
"The greatest sacrifice that God can ask of us is that of being treated as rebels while we want to serve the church as a mother in difficulty, overwhelmed, suffering," he said.
He also said his society and the Catholic Church spoke two "different languages".
Some 17,000 worshippers were in attendance at the ceremony, held entirely in Latin
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The Catholic Church spoke "the language of inclusion, dialogue, accompaniment", while his group was focused on "faith in all its simplicity".
Most of the ceremony - witnessed by 17,000 worshippers according to the group - was held in Latin.
Despite the group's traditionalist tendencies, the event had modern fittings, including scannable wrist bands used as digital wallets, QR codes on screens and baseball caps on sale as a souvenir.
Reverend Thomas Reese, a prominent Catholic author, said the ceremony showed how the society hoped to expand its influence.
He said: "They’re having a huge party. This is a marketing event. They want to be the leaders of the conservative dissenters in the Catholic Church, even if it means leading people out of the Catholic Church, which is what they’re doing."