China 'rotates' troops in Hong Kong ahead of planned rallies

China's military on Thursday said fresh troops had arrived in Hong Kong as part of a routine rotation, as the financial hub prepares for fresh political rallies against Beijing's tightening grip on the city.

State media published a video of armoured personnel carriers and trucks driving across the Hong Kong border in the border.

"The Hong Kong Garrison of the Chinese People's Liberation Army on Thursday morning completed the 22nd rotation since it began garrisoning Hong Kong in 1997," Xinhua news agency reported.

A new mass rally was planned for Saturday. But Hong Kong police denied permission for the demonstration on security grounds, raising the likelihood of another weekend of clashes between police and protesters.

In a letter to the rally organisers the Civil Human Rights Front (CHRF) on Thursday, police said they feared some participants would commit "violent and destructive acts".

Protesters have not only carried out "arson and large scale road blockades but also used petrol bombs, steel balls, bricks, long spears, metal poles, as well as various self-made weapons to destroy public property," the letter said of previous protests.

The protests were ignited when the city's Beijing-backed government tried to pass a bill allowing extraditions to mainland China.

But they have evolved into a wider call for greater democracy and an investigation into allegations of police brutality.

The mainly young protesters say their freedoms, unique within China, are being eroded by Beijing.

On Sunday police deployed water cannon and fired a warning gunshot to fend off radical protesters after a sanctioned rally turned ugly, in some of the worst violence of the past three months.

This Saturday's rally was set to mark five years since Beijing rejected political reforms in Hong Kong, a decision which sparked the 79-day Umbrella Movement.

The CHRF, responsible for the largest rallies the city has seen in decades, said they would appeal the decision.

"You can see the police's course of action is intensifying, and you can see (Hong Kong leader) Carrie Lam has in fact no intention to let Hong Kong return to peace," the group's leader Jimmy Sham told reporters.

Sham said he was attacked as he ate lunch by two "masked men armed with a baseball bat and a long knife", but escaped unhurt.

Anti-government demonstrators have been urged to gather in the city centre and march to the Liaison Office, the department that represents China's central government in Hong Kong, but both aspects, which need permission from authorities, have been banned.

The last event organised by the CHRF on August 17 brought hundreds of thousands of people to the streets.

It was a deliberate show of peaceful protest that saw demonstrators disperse without clashes.

On that occasion, the initial rally in a Hong Kong park was approved by authorities but protesters later defied a ban to march through the city.

The unrest has shown no sign of abating, with protesters locked in a stalemate with the Hong Kong government, which has refused to give in to their demands.

More than 850 people have been arrested since June.