Top Beijing officials in charge of Hong Kong affairs on Monday lashed out at critics of the city's new national security law, calling them "mantises and flies".

Hong Kong authorities last month enacted the financial hubs's second national security law, which expanded on legislation Beijing imposed four years ago to quell dissent after massive democracy protests in 2019 were quashed.

The two laws together punish nine categories of broadly-defined crimes -- ranging from sedition and insurrection to foreign interference and theft of state secrets -- with some carrying penalties of up to life imprisonment.

The latest law raised concerns over further infringement of human rights from the United Nations and several countries, including Britain , Australia, the United States and Canada.

It has also triggered discussions about Hong Kong's prospects as it charts a sluggish economic recovery post-Covid, even though officials say the law will usher in stability and prosperity.

On Monday, during the city's official celebration of China's National Security Education Day, a top Beijing official said it would act as "a guardian angel" for global investors, "protecting their rights, freedom, assets and investments".

"For an extremely small number of people who endanger national security, this law is an overhanging sharp sword," said Xia Baolong, Beijing's Hong Kong and Macau Affairs Office chief.

"Hong Kong's development would not be stopped by a few mantises and flies," he added in a speech from Beijing.

Speaking at the ceremony in Hong Kong, Zheng Yanxiong, Beijing's liaison chief in the city, declared "tit-for-tat cognitive warfare" against critics of the law.

"Some ill-intentioned foreign forces have been bad-mouthing China and Hong Kong... and even some renowned Western media has joined the wagon of slandering and smearing," Zheng said, adding "our only way to survive is to unite and fight".

Hong Kong authorities have sent at least seven letters to several foreign media outlets since March, condemning them for "misleading" reports on the new law.

US news outlet Radio Free Asia announced last month it had closed its Hong Kong office, citing concerns about staff safety, while media watchdog Reporters Without Borders said last week a representative was denied entry into the city.

More than 290 people have been arrested, 174 charged and 114 convicted -- most of them prominent pro-democracy politicians, activists, and journalists -- since Beijing's first security law was enacted.