Authorities worried about increasingly strident calls for Hong Kong independence are taking no chances ahead of a rare visit from one of China's top ranking officials, shutting down swathes of the city and reportedly gluing down pavers to quell the prospect of violent protests.

Mainland Chinese media have cited the visit by Zhang Dejiang, China's No. 3 and the first senior official to come since the 2014 Occupy democracy protests, as an example of Beijing's concern and support for the Asian financial hub.

Yet tensions are so palpable that thousands police have been mobilised to secure the city during Zhang's visit, which begins on Tuesday. Local media reported pavement bricks were being cemented to prevent them being used as missiles while police were camping atop a mountain where a pro-democracy banner was hung two years ago.

Independence, a taboo topic under both British and Chinese rule, has become increasingly mainstream subject in Hong Kong, with some activists calling for an outright breakaway from China, a move some politicians say would imperil Hong Kong's economic and political future.

"These young people have no idea that they could be putting Hong Kong on a potentially dangerous collision course with the motherland and bringing an unmitigated disaster," wrote former top Hong Kong security official Regina Ip in an editorial in the state-run China Daily.

"Separatism, or rather the anti-mainland doctrine in disguise, will...doom Hong Kong."

The young activists see it differently.

"(We) are facing a very great threat from China: Our culture, our language, our people...we are dying!" said Chan Ho-tin, the head of the newly-formed National party, expected to contest legislative elections in September.

"Do (Hong Kong people) want to be a Chinese city or do they want to be an independent country? There are only two choices."

Joshua Wong, another prominent young activist who launched a new political party called Demosisto this year, wouldn't rule out taking an independence line in upcoming campaigns.

"The problem with young people is that they are not 100 percent pre-occupied with economic considerations," said Michael Tien, a Hong Kong delegate to China's parliament, the National People's Congress, which Zhang heads.

"A lot of young people saying they don't want development, they want a better environment, they want better work-life balance, they want better quality of life."

"CLOSELY MONITORED"

Hong Kong guarantees freedom of expression under the agreement that saw Britain return its former colony to Beijing in 1997, but authorities haven't ruled out taking action against pro-independence activists.

"Any suggestion that (Hong Kong) should be independent or any movement to advocate such independence...would be inconsistent with the legal status of Hong Kong," the Department of Justice (DOJ) told Reuters.

The DOJ said it was watching for "possible criminal activities" and would "closely monitor the situation, maintain close liaison with the relevant law enforcement agencies, and take such action as may be necessary."

Hong Kong authorities said the "counter-terrorism security measures" were needed to ensure the safety of dignitaries during the visit.

China is an umbilical cord for Hong Kong's economy, with Chinese capital succouring financial markets and millions of Chinese visitors powering its tourism and retail sectors.

"Acts in favour of Hong Kong independence harm the sovereignty and security of the country, harm the prosperity and stability of Hong Kong, and harm the fundamental interests of Hong Kong," Chinese Foreign Minister spokesman Hong Lei told reporters on Monday.

Hong Kong also relies on China for food, water and electricity, making independence almost impossible in practice.

"A lot of people in Hong Kong have jobs associated with the mainland," said Holden Chow, vice-chairman of the DAB party, Hong Kong's largest pro-Beijing political party.

"If there are no more economic ties...then where are the jobs? There would be a rise in unemployment."

DON'T VOTE FOR 'THE EXTREMISTS'

While Hong Kong's independence movement is perhaps more a reflection of worsening political divisions than a realizable goal, the challenge to Beijing's authority is unnerving some.

Observers with close ties to Chinese officials say one of Zhang's priorities will be establishing relations with more moderate democrats to lower the heat.

"He will send a positive signal to any pan-democrat who is willing to have a dialogue with China," Tien said.

"This must be one of his key missions: To make sure the signal is strong enough that the electorate won't lambast the moderate pan-democrats and give all their votes to the extremists."