HONG KONG - Hong Kong's first bullet train glides out of a sleek harbourfront railway station bound for mainland China on Saturday (Sept 22), launching a new era of integration and raising fears for some for the territory's cherished freedoms.

Unlike other cross-border connections, the US$11 billion train project has stoked considerable controversy, with Hong Kong, a former British colony, having had to concede part of its jurisdiction to China for the first time.

Passengers entering the modernist building will have their documents stamped by Chinese immigration officers and will be subject to Chinese law while zipping across Hong Kong at 200 kph to the mainland.

Hong Kong and Beijing officials have justified the unusual arrangement as a one-off, while touting its logistical and economic benefits.

But critics say the railway is a symbol of continuing Chinese assimilation of Hong Kong, which returned to Chinese rule in 1997 with guarantees of widespread autonomy and freedoms not enjoyed on the mainland, including an independent legal system.

"I worry that Hong Kong will no longer be Hong Kong," said Martin Lee, a veteran democrat and barrister who is fighting to derail the project in the city's courts.

He said Hong Kong risked losing its allure as a financial hub underpinned by strong rule of law, as Beijing steps up a push to fuse the city into a vast hinterland of the Pearl River Delta including nine Chinese cities dubbed the Greater Bay Area.

Several other infrastructure projects will open this year including a US$20 billion sea bridge to Macau and Zhuhai, while Beijing is now working with Chinese tech giant Tencent to let people replace their travel documents by using a mobile phone WeChat app to cross the border in future.

"LITTLE ANT IN A BOX"

"Hong Kong will be completely submerged into the Greater Bay, and we don't know what it's going to be like," said the 80-year-old Lee, who helped found the city's Democratic Party.

"That's the worrying thing. Hong Kong is going to be put like a little ant into a box."

In recent years, especially since mass pro-democracy protests in 2014 that ended with no concessions from the government, Beijing has struggled to win hearts and minds, especially among those youths who reject any notion of being part of mainland China and have chafed at perceived meddling in Hong Kong's affairs.

As protesters demanded greater freedoms amid some talk of splitting Hong Kong from China, Beijing tightened its grip, jailing some young protest leaders, forcing others to flee and taking the wind out of the democracy movement's sails.

Hong Kong officials dismissed concerns that Hong Kong's identity was being swamped.

"In such a closely knitted hinterland, there is always a wish to bring down any sort of unnecessary barriers," Hong Kong Secretary for Commerce and Economic Development Edward Yau told Reuters.

"But that's not to say compromising on one country, two systems," Yau added, referring to the formula by which Hong Kong returned to Chinese rule with the promise that its "capitalist system and way of life" would remain unchanged until at least 2047.

"The bottom line will still be one country, two systems,"Yau added. "The Basic Law (Hong Kong's mini-constitution) does not have an expiry date of 2047."

Bernard Chan, who runs an insurance business and sits on Hong Kong's Executive Council which advises the government on policy, said he believes China wants to make Hong Kong work in its best interests.

The high speed rail and bridge will bolster existing infrastructure that already allows more than 600,000 people to shuttle between the two sides daily by road, rail and sea.

Beijing wants the Greater Bay Area, home to some 68 million people with a combined GDP of US$1.5 trillion, roughly that of Australia or South Korea, to model itself on San Francisco Bay and Tokyo Bay, to better meld people, goods and sectors including finance, tourism, logistics, manufacturing and tech.

"The truth of the matter is, of course, we are integrating fast, economically and socially," said Chan. "The one part that is not integrating at all is politics."