China has quietly more than doubled its deployment of mainland security forces in Hong Kong, according to foreign envoys and security analysts, in the most dramatic move yet by Beijing to prepare for a potential worsening of unrest in the global financial centre.

Key points: Analysts say China has assembled the largest ever active force of PLA troops in Hong Kong

Analysts say China has assembled the largest ever active force of PLA troops in Hong Kong Reinforcements include troops and equipment to quell urban violence

Reinforcements include troops and equipment to quell urban violence Protesters are planning to march on China's National Day

Last month, Beijing moved thousands of troops across the border into the city, which has been wracked by protests since June.

Chinese state news agency Xinhua described the operation as a routine "rotation" of the low-key force China has kept in Hong Kong since the city's handover from Britain in 1997.

Xinhua released this photo of a PLA boat arriving at a naval base on Stonecutters Island in Hong Kong last month. ( AP: Yuan Junmin/Xinhua )

A month on, seven Asian and Western envoys have told Reuters they are certain the late-August deployment was not a rotation at all, but a reinforcement.

Three of the envoys said the number of Chinese military personnel in Hong Kong had more than doubled since the anti-government protests began in June.

They put the number of Chinese military personnel at 3,000 to 5,000 in the months before the reinforcement, and estimated it was now between 10,000 to 12,000.

As a result, the envoys believe, China has now assembled its largest ever active force of People's Liberation Army (PLA) troops and other anti-riot personnel and equipment in Hong Kong.

Significantly, five of the diplomats say, the reinforcement includes elements of the People's Armed Police (PAP), a mainland paramilitary anti-riot and internal security force under a separate command from the PLA.

Until now, the presence of the PAP in Hong Kong has not been publicly known.

China's Ministry of National Defence, the PLA garrison in Hong Kong, the State Council Information Office, and the Hong Kong and Macau Affairs Office did not respond to questions from Reuters.

The office of Carrie Lam had no comment.

A Hong Kong police spokesperson told Reuters the police force was "capable of maintaining law and order and determined to restore public safety in Hong Kong".

After this report was published, Lawrence Li, spokesman for the Hong Kong Government's Security Bureau, issued a statement saying the PLA garrison was operating "in strict accordance" with the law.

"Details of the Garrison's rotation, including the number of its members involved, are defence matters," he wrote, and the Government "has no relevant information to provide".

The protests in the city erupted in response to a move by Ms Lam to propose a now-scrapped bill that would have allowed for the extradition of people from Hong Kong to China.

The demonstrations, which at times have turned violent, pose the biggest popular challenge to President Xi Jinping since he came to power in 2012.

The reinforcement in Hong Kong includes equipment tailor-made for the quelling of urban violence, including water cannon vehicles and trucks used to lay barbed wire barricades.

Reuters reporters have tracked increased activity at many of the PLA's 17 facilities across Hong Kong, Kowloon and rural New Territories, most of which were inherited under agreement with the departing British forces during the 1997 handover.

Some foreign analysts said China's reinforced military presence was bigger than expected.

"They do seem to have an active contingency plan to deal with something like a total breakdown in order by the Hong Kong police," said Alexander Neill, a Singapore-based security analyst at the International Institute for Strategic Studies.

'A day of grief'

On Sunday, violence erupted in Hong Kong for the second consecutive day, throwing the city's business and shopping belt into chaos and sparking fears of more ugly scenes leading up to China's National Day this week.

Protesters threw petrol bombs during clashes with riot police. ( AP: Vincent Yu )

Protesters are planning to march again on Tuesday despite a police ban, raising fears of more violent confrontations that could embarrass Mr Xi as his ruling Communist Party marks 70 years since taking power.

Posters are calling for October 1 to be marked as "a day of grief".

Hong Kong's Government has already scaled down the city's National Day celebrations, cancelling an annual fireworks display and moving a reception indoors.

Reuters