As the law has not even been drafted yet, it is hard to be concrete, but essentially people in Hong Kong fear the loss of their civil liberties. China expert Willy Lam is concerned the law could see people punished for criticising Beijing - as happens in mainland China. People are concerned this affect free speech their right to protest - which is currently legal in Hong Kong. In China, activities like this have been known to be classified as subversion.
Prominent activists such as Joshua Wong have been petitioning foreign governments to help their pro-democracy cause in the city. After years of lobbying, the US passed the Hong Kong Human Rights and Democracy Act. Some are worried that such campaign would constitute a crime in the future. Many are also afraid Hong Kong's judicial system will become like China's.
"Almost all trials involving national security are conducted behind closed door. It was never clear what exactly the allegations and the evidence are, and the term national security is so vague that it could cover almost anything," Professor Johannes Chan, a legal scholar at the University of Hong Kong, says.
Finally, people realise that an erosion of Hong Kong's liberties will affects its attractiveness as a business and economic powerhouse. It is not just its political but also its economic future at stake, observers say.