Hong Kong democracy advocates expressed alarm Friday after Chinese army vehicles were photographed travelling down a major thoroughfare, in what they condemned as a show of "military might" ahead of expected protests.

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.

At a small gathering of activists outside the city's legislature, surrounded by a ring of fences in anticipation of the upcoming protests, leaders from Occupy late Friday chanted slogans and vowed to press on with their movement if their demands are not met by Beijing.

"We are prepared to organise our protest actions, wave after wave," protest leader Benny Tai told reporters. "In the end we will have our final act of occupying the main streets of Hong Kong in Central."

China's state-run Xinhua news agency warned early Friday in a strongly worded article that the central government has "comprehensive jurisdiction" over Hong Kong and "will always be involved" in its affairs.

Story continues

"China will not squeeze Hong Kong's autonomy, but anti-central government groups should cast off the illusion that Hong Kong is under full autonomy," it added.

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.

- 'Creating fear' -

Occupy co-founder Chan Kin-man said the movement would not be cowed.

"The central government is intentionally creating fear in the community so that they can scare away our supporters," he told AFP.

Lai Tung-kwok, Hong Kong's security chief, vowed to crack down on anyone taking illegal direct action.

"The Hong Kong police and various departments have made all the necessary preparations," Lai told reporters. "The basic line is they cannot breach the law," he added.

Lai said he had "no knowledge" of PLA movements this week but added: "Since there are PLA stations in Hong Kong, the movement of the vehicles is a common phenomenon."

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 it is guaranteed civil liberties and freedom of speech unseen on the mainland.

China's Global Times tabloid, which has close links to the ruling Communist Party, said that "extremists" in Hong Kong cannot win out.

"The more the contention surrounding Hong Kong's political reform evolves into the confrontation between different forces, the less hope the pan-democratic camp has," it said in an editorial on Friday.

"The more they count on support from Washington and London, the more absolutely they will fail."