The office will implement a new law aimed at cracking down on dissent, terrorism and "collusion with foreign powers." In addition, bodies in all Hong Kong government departments will be directly answerable to Beijing.
The Chinese government is to establish a national security agency in Hong Kong, said state media on Saturday.
The National Security Defense Commission will oversee the implementation of a controversial new law aimed at cracking down on dissent in the semi-autonomous city that has been rocked by over a year of on-off pro-democracy protests and riots.
The office will also collect intelligence relevant to national security concerns and work with Hong Kong's judiciary to "handle criminal cases that threaten national security," wrote Chinese state news agency Xinhua.
Xinhua reported that, in addition, bodies in all Hong Kong government departments, from finance to immigration,
will be directly answerable to the central government in Beijing.
The new agency will be chaired by the city's chief executive and would be empowered to address security threats.
The chief executive will also get to appoint judges to try legal cases deemed to touch upon issues of national security, Xinhua added.
The announcement was made after a three-day meeting of China's top law-making committee, the Legislative Affairs Commission of the National People's Congress Standing Committee (NPCSC).
Controversial security bill
On the agenda was the controversial national security legislation that was proposed by mainland China's government in late May, sparking yet more pro-democracy protests.
Xinhua said the eventual law would criminalize "secession, subversion of state power, terrorism, and collusion with foreign forces" that endanger security. When the full draft is released, it will clarify the meaning of these, it added.
There will be increased "supervision ad management" of schools and social organizations on national security issues.
The legislation will override any existing Hong Kong laws that may conflict with it once it is implemented.
The standing committee would "soon finalize" the legislation, reported Xinhua, adding that the legislation has been fast-tracked so it bypasses Hong Kong's own legislature.
Bill met with opposition
The decision to pass the new security law was met by opposition, both from Hong Kong residents and the international community.
When first announced in May, protesters took to the streets to protest the proposed legislation.
This week all G7 nations signed a statement expressing "grave concern" about the law and urging Beijing to reconsider its plans.
A group of 86 international non-governmental organizations also issued a joint statement saying "China must abandon plans to introduce national security legislation" that, they say, "will threaten the basic rights and freedoms of people in Hong Kong."