HONG KONG, July 2 (AFP) — Hong Kong's Bar Association has said it is "gravely concerned" by Beijing imposing a sweeping security law on the financial hub, warning the broadly worded legislation undermines the city's independent judiciary and stifles freedoms.

"The Hong Kong Bar Association is gravely concerned with both the contents of the NSL (National Security Law) and the manner of its introduction," the influential legal association said in a statement released late Wednesday.

China imposed a sweeping security law on Hong Kong late Tuesday that dismantles the legal firewall that has existed between semi-autonomous Hong Kong's independent judiciary and China's party controlled court system since the 1997 handover.

The wording of the law was kept secret until it was enacted, outlawing acts of subversion, secession, terrorism and colluding with foreign forces. 

China says it will have jurisdiction over some cases and has empowered its security agents to operate inside Hong Kong openly for the first time, unbound by the city's laws.

On Wednesday, the first arrests were made, largely against people possessing flags and leaflets supportive of Hong Kong's independence. 

The law has sent a wave of fear through the city and rattled the legal community in a business hub that has built its reputation on the independence and reliability of its courts.

The Bar Association, which represents the city's lawyers, issued a scathing five-page legal analysis in which it highlighted more than a dozen areas where the new security law causes concerns.

It began by criticising the way Beijing kept the contents secret in a city that, until this week, used its own legislature to debate and enact legislation.

"In addition to the total absence of meaningful consultation, lawyers, judges, police and Hong Kong residents were given no opportunity to familiarise themselves with the contents of the new law, including the serious criminal offences it creates, before it came into force," the analysis said.