Britain returned the Asian financial hub to Chinese rule in 1997 with the guarantee of freedoms, such as an independent judiciary and right to protest, for 50 years.
"The imposition of the proposed law lies in direct conflict with China’s international obligations under the Joint Declaration, a treaty agreed by the UK and China and registered with the United Nations," Julian Braithwaite, Britain's ambassador to the U.N. in Geneva, told its Human Rights Council.
He urged China to engage with the territory’s people, institutions and judiciary to "ensure it maintains Hong Kong’s high degree of autonomy and rights and freedoms."
Jiang Duan, human rights minister at China's Geneva mission, took the floor to say that Braithwaite had "interfered in China's internal affairs which we firmly reject."
"China's enactment of laws in the Hong Kong special adminstrative region to safeguard national security is aimed at plugging loopholes...and effectively safeguarding national sovereignty and security," he said. "This is legitimate, legal and imperative."
Pang Kwang Hyok, deputy ambassador at North Korea's mission, voiced concern at "certain countries' attempt to use Hong Kong-related issues to interfere in China’s domestic affairs." Hong Kong is "an inseparable part" of where China's sovereignty is exercised and its constitution is applied, he said.
Hong Kong's national security legislation would not punish people retroactively, a senior Chinese official said on Monday, touching on a key question raised by local residents, diplomats and foreign investors over the disputed measure.
Beijing last month moved to directly impose the legislation on Hong Kong in a bid to tackle secession, subversion and foreign interference in the financial hub.
Hong Kong was rocked by months of sometimes violent anti-China, pro-democracy unrest last year, with protesters angry at what they see as meddling by Communist Party rulers in Beijing in Hong Kong's affairs.
China denies interfering and accuses Britain and the United States of fomenting the unrest.