Chinese Catholics pack Beijing's Cathedral of the Immaculate Conception for Sunday Mass, praying and singing hymns beneath stained glass windows much like their brethren around the world.

The spirituality on display, however, masks a complicated tangle encompassing the Vatican and clergy loyal to it on the one hand, and the officially approved Church in China, which does not recognise the authority of the Holy See.

The surprise retirement of Pope Benedict XVI and anticipation over his successor have brought back into focus conflict between a 2,000-year-old organisation that claims universal reach and China's Communist rulers.

Outside the state-sanctioned cathedral one worshipper epitomised the contradiction, pulling out a keyring carrying an image of John Paul II and saying that all Chinese Catholics support the popes.

Experts estimate that there are as many as 12 million Catholics in China, with about half in congregations under the officially administered Chinese Catholic Patriotic Association.

The rest belong to non-sanctioned or so-called underground churches, though despite the name many operate in the open, with experts saying levels of acceptance depend on local officials' attitudes.

The key point of contention is the approval of bishops, where the Vatican insists its authority is absolute.

Last July the state-run Church ordained a bishop in the northeastern city of Harbin in defiance of the Holy See, which subsequently excommunicated him. Chinese authorities dismissed the Vatican's protests as "rude and unreasonable".

In a dramatic display of defiance in the same month, Thaddeus Ma Daqin denounced the official Church at his own ordination ceremony as its auxiliary bishop of Shanghai, and has reportedly been under house arrest ever since.

"What we desire is a wonderful relation between the Holy See and China and the unity of the Church in China but in this mood they are making it impossible," Cardinal Joseph Zen, former leader of Hong Kong's Roman Catholics, told AFP.

During his tenure, Benedict sought a measure of accommodation, assuring Beijing that the Vatican had no intention of undermining its rule and offering encouragement to Catholics in official congregations.

The Catholic Church in China "does not have a mission to change the structure or administration of the state", he made clear in a 2007 letter to Chinese Catholics.

But he drew a line at any governmental interference in the Vatican's right to manage ecclesiastical affairs and insisted that all lay Catholics were members of a single global Church -- a challenge to Beijing's self-declared powers.

"The solution to existing problems cannot be pursued via an ongoing conflict with the legitimate civil authorities," Benedict wrote.

"At the same time, though, compliance with those authorities is not acceptable when they interfere unduly in matters regarding the faith and discipline of the Church."

The Chinese Communist Party is officially atheist and heavily persecuted religious believers in its early decades in power. It doggedly guards its dominance of public life and remains deeply suspicious of any possible foreign political influence.

It is a difference as intractable as a theological schism, but despite the tensions, popes are not vilified by China's government and state media, unlike the Dalai Lama of Tibetan Buddhism, who is depicted as a threat to the nation.

Nonetheless, after Benedict's surprise resignation announcement, China said improved relations were only possible if the Vatican took "a flexible and practical attitude", switched diplomatic recognition from Taiwan to Beijing and, crucially, minded its own business.

"The Vatican should not interfere in China's internal affairs, including interferences in the name of religious affairs," said foreign ministry spokesman Hong Lei.

The underground Church says it will stand firm on its principles under the new pope, while building on dialogue with the Chinese government.

"We will still say what we need to say and we will still do what we need to do," an underground priest said in an email to AFP delivered via the Justice and Peace Commission of the Hong Kong Catholic Diocese.

If the Church allowed political considerations to override its theological mission "our loss will be very, very great," he warned, speaking anonymously because of concern for his safety.

The dispute leaves the country's Catholics -- a minority among the estimated 67 million Christians of all denominations -- feeling vulnerable.

"China's Catholics remain nervous that their situation could change at any time," said Anthony E. Clark, an expert on the faith in the country, who teaches Chinese history at Whitworth University, a Christian institution in the United States.

"Recent months have seen a tight government crackdown on underground Protestant churches... and Catholics worry that more state interference might be coming their way too."

In Shanghai, visitors to the hillside pilgrimage site of Sheshan Basilica had divided views on prospects under Benedict's successor.

"Everything is possible," said Marco Ju, who converted to the faith four years ago.

But a priest in the state-sanctioned Church who identified himself as Father Fang said he doubted the new pope could normalise relations.

"This target will never be achieved," he said. "The church will not give up its independence."