The link, which cost more than $10bn and took more than eight years to build, will vastly decrease travel times between the two sides.
The system aims to transport more than 80,000 passengers daily between the Asian financial centre of seven million people and the neighbouring manufacturing hub of Guangdong province.
Passengers will clear Chinese immigration at the line’s newly built West Kowloon terminus, the source of major legal controversy when it was revealed that mainland Chinese law would apply within roughly one-quarter of the station’s area.
Al Jazeera’s Sarah Clarke, reporting from Hong Kong, said the project attracted criticism for being behind schedule and expensive.
“It has not been smooth sailing. A decision to put joint immigration checkpoints in Hong Kong, not at the border, angered pro-democracy groups,” said Clarke.
Au Hok-hin, a protest organiser, opposed China setting up police stations in the new terminal.
“There are ambiguities between the jurisdictions of Hong Kong police and mainland China police,” said Au.
The opposition MPs argued the move to open joint checkpoints in Hong Kong would be a violation of the Basic Law, Hong Kong’s mini-constitution under which it retained its own legal system and civil liberties after reverting from British to Chinese rule in 1997.
That guarantees Hong Kong the right to maintain rights such as freedom of speech and assembly – which are routinely violated on the mainland – until 2047.
Legal matters related to defence, foreign affairs and national security fall under Beijing purview.
— Tony Sabine (@chopkickpunch) September 22, 2018
However, China’s tight control over the city’s politics and a continuing crackdown on politicians calling for greater economy and democratic reforms have spurred worries about an erosion of Hong Kong’s remaining autonomy.
The Hong Kong legislature’s passage in June of the plan to allow Chinese law to apply at the railway terminus was a significant moment for the opposition, coming four years after mass street protests demanding reforms fizzled out amid Beijing’s intransigence.
Supporters of the provision, including the territory’s Beijing-backed Chief Executive Carrie Lam, defended it as promoting speed and convenience.