China will have overarching powers over the enforcement of a new national security law in Hong Kong, according to details released on Saturday that signalled the deepest change to the city's way of life since it returned to Chinese rule in 1997.

The planned law has alarmed foreign governments as well as democracy activists in Hong Kong, who were already concerned that Beijing is eroding the high degree of autonomy granted to the territory when it was handed over from British rule.

According to details released by the official Xinhua news agency, Hong Kong will establish a committee to safeguard the legislation, headed by the city's leader Carrie Lam and supervised by the central government.

New police and prosecution units will be set up to investigate and enforce the law.

Lam will also have the power to appoint judges to hear cases related to national security, an unprecedented move likely to unnerve some investors, diplomats and business leaders in Hong Kong Currently senior judges allocate judicial rosters up through Hong Kong’s independent judicial system.

“From these initial details, this new law presents unprecedented legal questions that we will have to confront in coming years,” Simon Young, a barrister and professor at the University of Hong Kong’s law school, told Reuters.

Young said he was troubled by the apparent “broad supremacy” of the new law over current and future Hong Kong laws.

Officials in Beijing and Hong Kong have sought to reassure investors that the law will not erode the city’s autonomy, insisting it will target only a minority of “troublemakers” who pose a threat to national security.

Xinhua said human rights and freedom of speech and assembly would be protected, echoing previous comments by authorities in Beijing and Hong Kong.

The details were unveiled after a three-day meeting of the top decision-making body of China’s parliament. It is unclear when the law will be enacted but political analysts expect it to take effect ahead of Legislative Council elections in Hong Kong on September 6.

China, which sees a growing national security threat from Hong Kong, says the draft law is aimed at tackling separatist activity, subversion, terrorism and collusion with foreign forces.

Critics fear it will crush wide-ranging freedoms that are seen as critical to Hong Kong's status as a global financial centre.

Under the details released on Saturday, any Hong Kong resident running for election or working for the government will have to swear allegiance to the city and its mini-constitution, the Basic Law.

China’s move to impose the law directly on Hong Kong, bypassing the city’s legislature, comes after a year of sometimes violent anti-government and anti-Beijing protests. Mainland authorities have blamed “foreign forces” for fomenting the unrest.

Some commentators say the law is aimed at sealing Hong Kong’s “second return” to the motherland after the handover from Britain failed to bring residents to heel. At the time of the handover, China promised to allow Hong Kong a high degree of autonomy for 50 years under what is known as the “one country two systems” formula of governance.