BEIJING - A group of Christians gathered in an apartment above a Beijing dental surgery, the atmosphere jubilant as a choir belted out carols on Christmas Eve -- but the curtains stayed tightly closed.
Unofficial Christian groups have long been subject to crackdowns, but the atmosphere appears to be worsening as their numbers increase, and the ruling Communist party takes a more nationalist tone under its leader Xi Jinping.
Members of Shouwang, a Beijing Christian group who held the Christmas Eve service, have faced more trouble than most.
Several pastors from the group -- which at its height boasted around 1,000 mainly middle-class members -- have been under house arrest since they tried to arrange Easter services in a public square in 2011.
Nonetheless there was a joyful atmosphere in the 12th storey apartment this Christmas where green and silver tinsel hung beside plastic snowflakes, and several dozen worshippers joined in with Chinese versions of traditional carols "Away in a Manger," and "Noel, Noel."
"Things have got worse this year because the police started to detain us. I was detained for a week," said Zhao Sheng, 54, musical organiser for the service.
"But Christmas is still a happy time. No matter what happens, God is with us," he added with a grin.
Several of the group's pastors have been kept under house arrest since they tried to arrange Easter services in a public square in 2011.
You Zhanglao, one of those under house arrest, said in a telephone interview that he had celebrated Christmas "at home with my family by saying prayers".
Christianity has aroused suspicions in China since the 19th century when it was spread by foreign missionaries who often worked alongside colonial European powers.
China's ruling Communist party is officially atheist and effectively banned the religion during the 1960s, but the Christian population has swelled at rates of up to ten per cent each year since restrictions were relaxed around thirty years ago.
The country is now home to an estimated 70 million Christians, according to a 2011 survey by the Pew Research Center, as people search for a sense of community and meaning in a fast-changing society.
The vast majority of Chinese Protestants -- around 50 million, according to the survey -- shun state-run churches and worship in self-organising groups outside government-control.
These underground churches are technically illegal, giving authorities a pretext to crack down if they wish.
Local authorities who have long tolerated underground churches have taken a harder line this year.
The summer in the eastern city of Wenzhou, sometimes known as "China's Jerusalem," because of its large Christian population, police stormed Churches to force the removal of visible crosses.
The crackdown affected more than 400 churches in eastern Zhejiang province, according to US-based rights group China Aid, with some churches completely demolished.
In the strongest sign of official fears so far, top religious official Wang Zuoan told worshippers at Beijing churches to "resolutely resist the use of of Christianity to by foreigners to infiltrate China," according to the state-run China News Service.
Experts say the Christian conception of universal values fits uncomfortably with the Communist party's insistence that China cannot be judged by foreign standards.
Richard Madsen, expert on Chinese Christians at the University of California San Diego, said "there seems to be a new move to try and suppress Churches."
"Its connected with the nationalism of China's government, and concerns that this is a foreign religion with connections around the world," he added.
Since coming to power in 2012, China's President Xi Jinping has praised the ancient Confucian moral system, and previously met top Buddhist figures.
China's leaders "hope the revival of traditional religions like Taoism and Buddhism will help crowd-out Christianity," Madsen said.