The Vatican has issued a strong denunciation of religious repression in China because of fears that 23 Roman Catholics - including eight priests - have been arrested, and some ordered to undergo enforced re-education.
One aged bishop is understood to have died in prison.
The Pope's spokesman, Joaquin Navarro-Valls, criticised China for arresting the eight priests and two seminary students in Hebei province in August.
"The reasons for such repressive measures have not been made known to the Holy See," he said.
"If the received news turns out to be true, we find ourselves once again faced with a grave violation of freedom of religion, which is a fundamental right of man."
Two of the priests were sentenced to "a period of re-education through forced labour," while others, detained in Baoding diocese, had not yet been released, Mr Navarro-Valls said in a statement over the weekend.
So far, the Vatican has not been told the reasons for the arrests.
"According to the information received here, as of September 6, 2004, the number of clergy members of the Baoding diocese detained or deprived of liberty is 23, including a bishop and his auxiliary who disappeared in 1996 and 1997," he said.
The reported arrests are the latest in a string of detentions of Catholic priests and bishops, some of whom run underground seminaries in unofficial churches and private homes, remaining loyal to the Vatican in a communist state which does not recognise the Pope's authority.
The arrests reportedly took place on August 6 at a religious retreat in a village in Hebei province, where many of China's unofficial Roman Catholics live.
Monsignor Giovanni Kexian, Bishop of Yantai, in Shandong Province, died in prison aged 76, the Vatican said.
The bishop had been arrested in the late 1990s for refusing to adhere to the Communist party's demand that Catholics worship only in churches approved by a state-controlled church group which does not recognise the Pope's authority.
Bishop Kexian was delivered to his family in a coffin with no explanation, the Vatican statement said.
Since China broke diplomatic ties with the Holy See in 1951, the Vatican has preferred to resolve alleged cases of religious repression through diplomatic channels.
Despite the official ban on independent Catholic worship, the US-based Cardinal Kung Foundation estimates there are 12 million "underground" Catholics, compared with the 4 million who follow the state-authorised church.
Recent developments in China point to increasing crackdowns on Chinese Buddhists, Christians and practitioners of Falun Gong.
On August 11, the Chinese authorities reportedly arrested Yu Tianjian, a prominent Chinese Buddhist who also maintains residency in the United States.
Mr Yu had renovated a Buddhist temple in the Chinese province of Inner Mongolia.
At his arrest, he said that he had been charged with "promoting superstition".
Falun Gong claims that more than 800 of its members have died in custody since the movement was banned in 1999.
This year, as debate has centred on China's human rights record as the country prepares to host the 2008 Olympic games, the Vatican has hardened the tone of its public complaints.
The Vatican's public offensive comes as the first Chinese tourists have begun visiting the seat of the Roman Catholic church after September 1, when an agreement came into effect allowing Chinese tourists to visit Europe. Previously only business groups were allowed.