Over four balmy nights outside Hong Kong's main government building, 28-year-old Freeman Leung sat on the ground among fellow Christians and sang a hymn over and over.

Key points: The 1970s American Easter hymn has been sung during Hong Kong's protests against the China extradition bill

The 1970s American Easter hymn has been sung during Hong Kong's protests against the China extradition bill Many of Hong Kong's top human rights dissidents are Christians

Many of Hong Kong's top human rights dissidents are Christians In Hong Kong there have long been links between pro-democracy activists and the Catholic Church

It was the same hymn that Christians in the Chinese territory had been singing for the past two weeks.

Sometimes they were joined by non-believers at protest rallies both large and small.

"Christians started turning up at protests to sing 'Sing Hallelujah To The Lord' in case there was the chance of violence when police wanted to disperse protesters," Mr Leung said.

"But once they started singing, everyone became calm."

Christian activist Freeman Leung said the hymn had a calming effect on the protests. ( ABC News: Craig Berkman )

Hong Kong's recent protests have waxed and waned between extraordinary street marches of up to 2 million people and days where barely 100 people turned up.

But throughout it all, the same 1970s American Easter hymn has been resonating through the demonstration sites.

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The unlikely protest anthem has even been embraced by non-religious protesters.

"It's a very simple hymn, everyone can sing it," said Edwin Chow, the chairman of the Hong Kong Federation of Catholic Students.

Christian groups are not taking credit for the uprising against a proposed bill which would have allowed Hong Kong citizens to be sent to mainland China for trial.

Religious demonstrators are still singing outside the legislative council building in Hong Kong. ( ABC News: Craig Berkman )

But Christian demonstrators have had a constant presence, which some other demonstrators likened to protection.

"Some non-Christians have been singing 'Hallelujah' too because in Hong Kong, a religious gathering can't be deemed by police as an illegal assembly," Mr Chow said.

"Through singing the hymn, you can see it helps protect other demonstrators."

Hong Kong's Christians fear extradition to the mainland

Religious groups in Hong Kong have extra incentive to oppose the city government's plan to send suspects to face Communist party-controlled courts on the mainland.

About 12 per cent of Hong Kong's population are Christian. ( ABC News: Craig Berkman )

China's constitution supposedly safeguards religious freedom.

But the Communist Party has launched sporadic crackdowns on churches, including a series of demolitions in recent years carried out by local governments across China.

Many of the country's top human rights dissidents are Christians, including prominent jailed lawyer Wang Quanzhang and the exiled activist Chen Guangcheng.

Political leaders on the mainland must eschew signs of religious beliefs to affirm their atheist credentials.

A Hong Kong law which allows religious gatherings was used to protect demonstrators from violence. ( ABC News: Craig Berkman )

Other religious groups — particularly Muslims in the country's far-west — have been the subject of highly expansive government campaigns aimed at ensuring the loyalty of believers is with the Communist Party first.

In Hong Kong there have long been links between the pro-democracy activists and in particular the Catholic Church, which has a decades-long unresolved dispute with China's Government over the right to ordain bishops.

The city's most prominent young political activist, Joshua Wong, is a devout Christian, as are many older members of the pan-democratic camp.

"Some Christians, including me, are afraid that if the extradition bill is passed, it could affect freedom of religion in Hong Kong and freedom of religious activities," Mr Chow said.

Many Christian activists in Hong Kong fear they could be extradited to the mainland if the bill is passed. ( ABC News: Craig Berkman )

He believes it is this fear that has mobilised a larger-than-normal turn-out among the city's Christians, who number around 900,000 — or about 12 per cent of the population.

But there have been rumblings among some younger protesters that church leaders, particularly Protestants, have refrained from adding their voices to the movement because they are too close to the pro-Beijing establishment.

Lina Chan from the Justice and Peace Commission of the Hong Kong Catholic Diocese said churches are divided over what role they should take.

Hong Kong Christians are divided over what role churches should play in social movements. ( ABC News: Craig Berkman )

"Some believe they should be taking a leading role, but others think they should be more balanced and non-political," Ms Chan said.

"On this issue, there have been more people urging the church to be active and speak up."

One of the Protestant churches in the city that organised prayer events was Hong Kong's Methodist Church, which opened its doors late for protesters who wanted to rest.

"In the past, a lot of people say we Christians just hid ourselves in the church, but actually we are quite involved and quite concerned about the things happening in our society," said Douglas Lee, one of the event organisers.

He believes the past few weeks have been an awakening — even for believers who usually shy away from politics.