LONDON (Reuters) - Hong Kong's fate shows China's true authoritarianism so the rest of the world should stand up to President Xi Jinping and start to put human rights above financial gain, pro-democracy activist Nathan Law told Reuters.

China unveiled a national security law this week which Hong Kong pro-democracy protesters and the West says breaches the "one country, two systems" principle enshrined in the 1984 Sino-British treaty that guaranteed the autonomy of Hong Kong.

"The protests in Hong Kong have been a window for the world to recognise that China is getting more and more authoritarian," Law told Reuters via internet video. Law, 26, left Hong Kong this week. He declined to disclose his location.

Law called on the international community to put human rights above the financial interests of trading with the world's second largest economy and to "multilaterally deal with the China issue in a more assertive way."

(Reporting by Natalie Thomas; editing by Guy Faulconbridge)