Chinese Catholics flocked to Mongolia's capital to see Pope Francis this weekend, getting a glimpse of grand public expressions of religious faith unthinkable back home, and visitors expressed hope that the pontiff could one day visit them.

China is home to some 12 million Catholics, who for decades found themselves torn between Communist party-approved worship and underground churches backed by the Vatican.

And this weekend, with the pope on their doorstep for the first ever papal visit to Mongolia, some journeyed to see him for themselves.

In Ulaanbaatar's central square, AFP saw a Chinese flag unfurled as supporters gathered to see the pope -- several wearing masks and sunglasses that concealed their faces.

One visitor said she and her companions had to be "very low-key".

"We were asked at customs why we are (going abroad) and whether we are Catholics. We said we were here for travel," she said.

There is "a lot of pressure on Catholics in China", she explained. 

"We also worry that when we return, we will be invited for 'chats'," she said, using a common euphemism for being summoned for questioning by state security officials. 

The visitor said she and a group of about 20 had come from across northern China to see the pope.

"We dare not to make our trip known to others."

The Communist Party is officially atheist and exercises strict control over all recognised religious institutions, including vetting sermons and choosing bishops.

Pope Francis led a years-long effort to build ties with Beijing and in 2018 the Holy See reached a secretive agreement allowing both sides a say in appointing bishops in China.

Visitors in Mongolia told AFP of hopes that ties could warm further -- and that the pope could even one day visit.

"I hope that the pope comes to China," a 75-year-old Catholic surnamed Kong -- who said she has raised 48 orphans by herself -- told AFP.

Both sides "should talk diplomatically", she said.

Tamir Amarjargal, a 26-year-old tourist from China's Inner Mongolia, while not a Catholic, expressed similar hopes.

"It's very rare for the pope to visit Asia," he said.

- 'Very excited' -

"Mongolia is so close to us, so I'm here to see him. There are quite a few Catholics in my group, who are very excited to see the pope for the first time," he explained.

"I hope he could visit China one day. That would be even better."

Lu Lei, 38, a petroleum engineer from China's northern Heilongjiang province, told AFP he was interested in Catholic culture, though not a believer himself.

He said he had watched the TV drama "The Young Pope" -- a satirical series about a fictional conservative American pontiff, played by Jude Law -- in preparation for the visit.

"Definitely I hope the pope will visit us in China too," he said.

"Every country should support different religions and let its people choose their own religion freely."

China's constitution does guarantee religious freedom.

But rights groups say religious organisations face routine persecution and that freedom to worship is severely curtailed -- a trend they say has worsened under President Xi Jinping's rule.

"We can't enter churches on sensitive days, like Easter, Christmas Day" due to intimidation from authorities, one Chinese visitor in Ulaanbaatar said.

The authorities say "one thing, and do another", she said.

AFP heard one visitor advising another not to speak with reporters, for fear of "trouble" upon their return to China.

"I think you know why we aren't able to say much," one woman waiting at the cathedral said.

"You know how it is."

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