Pope Francis called on Chinese Catholics on Wednesday to affirm their loyalty to him and spread the word of God, and prayed that they be allowed to live in accordance with their faith.

"I invite the Catholics of the world to come together in prayer with their brothers and sisters in China for the grace to humbly and joyfully announce Christ... and be faithful to his Church and to Peter's successor," Francis told 50,000 pilgrims at a weekly audience in St Peter's Square.

The pope also called for prayer so that Chinese Catholics "can live their lives in the service of their country and their fellow citizens, in a way that is consistent with their faith."

Francis prayed to the Virgin Mary to give strength to Chinese Catholics ahead of a religious celebration on Friday, which draws fervent pilgrims to the Sheshan shrine in Shanghai in particular.

There are some 5.7 million Catholics in China according to official figures, though independent experts say there are closer to 12 million.

They are caught between staying loyal to the ruling Communist Party in Beijing and belonging to an "underground" Church not recognised by the authorities, and paying allegiance to the pope.

Beijing and the Vatican have been at loggerheads since China severed ties with the Holy See in 1951. The atmosphere worsened when in 1957 China set up its own Catholic Church administered by the atheist Communist government.

In March, China said it hoped the newly elected Francis would take a "practical and flexible" attitude, warning the Vatican against "interfering in China's internal affairs, including under the pretext of religion".

More than 50 million Christians in China are believed to pray at "underground" or "house" churches, which refuse to submit to government regulation.