At least four People's Liberation Army (PLA) armoured personnel carriers were seen in the small hours of Thursday near the busy Jordan and Yau Ma Tei regions of the city, the Apple Daily newspaper reported.
The vehicles, with short guns mounted on turrets, were spotted at a time of heightened public discontent in the semi-autonomous city over perceived interference by Beijing and a debate over how the next chief executive will be chosen under planned reforms.
Beijing has promised the former British colony will be able to vote for its own leader in 2017.
But it has insisted on vetting candidates through a pro-Beijing nominating committee, a move activists fear would disqualify anyone critical of the mainland authorities.
A pro-democracy group, Occupy Central, has pledged to mobilise thousands of protesters to block the financial district if authorities refuse to allow the public to choose candidates.
Organisers plan to hold a rally on Sunday when the top committee of China's rubber-stamp legislature is expected to announce its decision on what form the political changes will take.
Pro-democracy lawmaker Claudia Mo said she believed the movement of the armoured carriers was a deliberate attempt to frighten activists ahead of the protests.
"It's a show of military might to scare off Hong Kong people who are about to stage some large-scale civil disobedience activity. The timing is very suspicious" she told AFP.
Occupy co-founder Chan Kin-man said the movement would not be cowed.
The Hong Kong government declined to comment on the sightings while the PLA did not immediately respond to AFP inquiries.
The Chinese army and navy have bases in Hong Kong but have generally kept a low profile ever since the former colony was handed over to China in 1997.
Tanks are often viewed by Hong Kongers as a symbol of Beijing's autocratic tendencies.
During frequent pro-democracy protests in the city, activists often make homemade tanks in reference to the famous "Tank Man" photograph taken during the bloody Tiananmen Square crackdown of June 4, 1989.
"Oh no, its really going to be a copy of June 4th," Teresa Leung, an Apple Daily reader, commented on their report.
"If they use force to suppress Occupy Central, the result would be unimaginable," Tina Ho added on the paper's website.
The city was handed over under an arrangement where the city is guaranteed civil liberties and freedom of speech unseen on the mainland.
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