Authorities in northern China detained two priests from the underground Roman Catholic Church as they were about to begin classes on natural birth control and theology, a Connecticut-based lobby group said Monday.

Police in Dingzhou, a city in Hebei province, refused to comment on the Cardinal Kung Foundation's (search) report that they detained Lu Genjun, 42, and Cheng Xiaoli, 40.

The Stamford, Conn.-based foundation provides support to the Catholic church in China and monitors the communist government's efforts to suppress it.

Lu, ordained in 1990, was recently released from a labor camp after spending three years there. The report said he was detained in Dingzhou. Cheng's whereabouts were unknown.

China broke ties with the Vatican (search) in 1951 and demands that Catholics worship only in churches approved by the China Patriotic Catholic Association, a state-controlled body that does not recognize papal authority.

Many Chinese Catholics, however, remain loyal to the pope and risk arrest by worshipping in unofficial churches and private homes. The state church claims 4 million believers, but the Cardinal Kung Foundation said the unofficial church has 12 million followers. Many believers and some priests move between the two churches.

Hebei province, next to Beijing, is a stronghold of Catholic sentiment. A number of priests there have been detained, some for years.

In a separate report, the New York-based Committee for Investigation on Persecution of Religion in China (search) said Xu Shuangfu, founder of a Protestant evangelical group, was kidnapped last month while visiting the northeastern province of Heilongjiang.

Dozens of church members have been arrested in a police crackdown, and one was beaten to death, the Committee said.

Police in Heilongjiang would not comment on the report.

Protestants also are required to worship in state-sanctioned churches, and independent church organizers and worshippers are routinely harassed and detained.