Beijing has faced a barrage of criticism over the law, which was imposed in late June after pro-democracy protests rocked the semi-autonomous city last year.

The law, which criminalises secession, subversion, terrorism and colluding with foreign forces, carries a maximum life sentence and has intimidated many protesters into silence.

In a letter made public Friday, the UN advisers warned parts of the legislation "appear to criminalise freedom of expression or any form of criticism" of China.

"The National Security Law... poses a serious risk that those fundamental freedoms and due process protections may be infringed upon," the rapporteurs said.

The letter warned the legislation may "impinge impermissibly on the rights to freedom of opinion, expression and of peaceful assembly."

The rapporteurs urged China's "reconsideration" of the legislation and for a fully independent reviewer to be appointed to ensure it complies with China's international human rights obligations.

They also expressed concern over one of the most controversial points of the law -- which allows cases can be transferred from the jurisdiction of Hong Kong to mainland China -- and warned it could undermine the right to a fair trial.

Critics believe the security law has ended the liberties and autonomy that Beijing promised Hong Kong could keep after its 1997 handover by Britain -- freedoms unique within China.

The broadly worded law criminalised certain political speech overnight, such as advocating sanctions, and greater autonomy or independence for Hong Kong.

Lawyers acting for some of the more than 20 people arrested under the law so far say police are trawling through historic actions of pro-democracy activists to beef-up their cases.

The UN experts also raised concerns over the definition of terrorism under the national security law.

They warned it extends to damage of physical property such as transport facilities -- which goes well beyond the UN Security Council's definition of terrorist conduct as aiming to cause death or serious bodily harm.

Since the law was imposed on Hong Kong, a raft of nations have ended bilateral deals with the city, including extradition treaties with the US, Britain, Canada, Australia, France and Germany.

The law has also sparked fears over plunging press freedoms, particularly after the arrest of a pro-democracy media tycoon under the new law last month.