HONG KONG (AFP, REUTERS) - A sweeping national security law that will soon be imposed on Hong Kong will be "like installing anti-virus software", a Beijing official in Hong Kong said on Monday (June 8), in a speech that warned democracy protesters had gone "too far".

The comments by Mr Zhang Xiaoming, deputy director of the Hong Kong and Macau Affairs Office, were the most detailed from a senior party cadre since Beijing announced plans last month to outlaw subversion, secession, terrorism and foreign interference.

His remarks came a day before the restless city marks one year since huge and often-violent protests erupted, raging for seven straight months in the most direct challenge to Beijing's rule since the city's 1997 handover.

"Once in force, this law will be like installing anti-virus software into Hong Kong, with 'one country, two systems' running more safely, smoothly and enduringly," Mr Zhang said, referencing the model by which China allows Hong Kong certain freedoms and autonomy denied to its citizens in the mainland.

Opponents fear the law - which is currently being drafted in Beijing and will bypass Hong Kong's legislature - will bring political oppression to a business hub supposedly guaranteed freedoms and autonomy for 50 years until 2047.

On the mainland, anti-subversion laws are said to be used to stamp out dissent.

During his speech, Mr Zhang repeated Beijing's assertions that the law would only target an "extremely small number of people".

"The opposition camp radical separatists have been mistaking the central government's restraint and forbearance for weakness and timidity," he said. "They have gone too far."

Millions of Hong Kongers hit the streets last year during the months of rallies, triggered by the now scrapped extradition Bill that could have sent people to mainland China for trial.

The protests were viewed as the culmination of years of rising fears among Hong Kong residents that Beijing was prematurely eroding the city's freedoms.

But Beijing has portrayed the protests as a plot by foreign powers to destabilise mainland China.

"The opposition camp... wants to turn Hong Kong into an independent or semi-independent political entity, a bridgehead for the external powers to oppose China and the Chinese Communist Party and a chess piece which external powers can use to contain China," Mr Zhang said.

During last year's protests, Mr Zhang's office and Chinese state media initially said issues like a lack of housing and the high costs of living may have fuelled the unrest.

But in recent months Beijing has instead cast the city's political crisis as a national security threat.

"From my point of view, the key problem in Hong Kong is not an economic problem, nor a livelihood problem concerning people's housing and employment... It is a political problem," Mr Zhang said.

The planned law approved by China's Parliament has also proposed allowing mainland security agents to set up shop in Hong Kong for the first time.

Mr Zhang dismissed "rumours" they might make arrests and send suspects to the mainland.

"National security organisations have to follow the laws strictly when they are handling cases in mainland China, how is it possible for them to become unconstrained in Hong Kong?" he said.

In rare comments on Hong Kong's long-term future, Mr Zhang, speaking via a videolink to an online seminar about the city's mini-Constitution, indicated that how Hong Kong's people behaved now in regard to the political situation would affect its post-2047 status.

He said the stronger the commitment to national security, the more room there would be for the "One Country, Two Systems" arrangement.