Protesters sing religious songs outside the Legislative Council building in Hong Kong. (Reuters)

The Christian hymn "Sing Hallelujah to the Lord" has emerged as the unlikely anthem of Hong Kong's protests against an extradition bill that have drawn millions of people onto the streets.

Protests around the world often develop their own soundtrack, usually songs with lyrics of defiance and solidarity, aiming to keep crowds energised and focused.

But the hymn taken up in Hong Kong hardly ticks those boxes.

For the past week, the hymn has been heard almost non-stop at the main protest site, in front of the city's Legislative Council, and at marches and even at tense stand-offs with the police.

It started with a group of Catholic students who sang several Christian songs at the main protest site, with "Sing Hallelujah to the Lord" catching on among the crowd, even though only about 10 percent of Hong Kong people are Christian.

"This was the one people picked up, as it is easy for people to follow, with a simple message and easy melody," said Edwin Chow, 19, acting president of the Hong Kong Federation of Catholic Students.

Protesters sing religious songs outside the Legislative Council building in Hong Kong. (Reuters)

The students sang the songs in the hope of providing a cover of legitimacy for the protest. Religious gatherings can be held without a permit in the financial hub.

"As religious assemblies were exempt, it could protect the protesters. It also shows that it is a peaceful protest," Chow said.

The hymn was composed in 1974 by Linda Stassen-Benjamin in the United States for Easter. Its five words are repeated over four stanzas in a minor key, which gives it an air of meditative solemnity.

The protests over the past 10 days have been largely peaceful although police on Wednesday last week used tear gas and rubber bullets to disperse the crowds.

"Stop shooting, or else we sing 'Hallelujah to the Lord'," read one protest placard after the rubber bullets were fired.

Protesters say the religious song has often helped defuse tension with the police.

"It has a calming effect," said Timothy Lam, 58, a Catholic priest at Grace Church Hong Kong, 58-year-old Lam, who has attended the protest with other churchmen to promote peace.

"The police had a lot of equipment, they were very tense and searching people. The students sang this to show they were peaceful," Lam said of a confrontation last week.

Hong Kong's Beijing-backed government leader, Carrie Lam, has postponed the introduction of the extradition bill and apologised in the face of the huge show of opposition.

Critics say the law would undermine Hong Kong's independent judiciary and rule of law, guaranteed by a "one country, two systems" formula under which the former British colony returned to Chinese rule in 1997.

Carrie Lam is Catholic and some protesters said they thought their adoption of "Sing Hallelujah to the Lord" might have helped sway her.

"She is Catholic after all, it's one of the main reason we sing it," said Jamie, an 18-year old student who is not Catholic.