Exiled Hong Kong activist Nathan Law on Thursday presented his first English-language book, "Freedom", warning of the global threat from Chinese authoritarianism.
Law, 29, was one of the most prominent figures in the Umbrella Movement pro-democracy protests in the former British colony in 2014.
He was elected as the youngest lawmaker in Hong Kong's Legislative Council at 23, only to be swiftly disqualified and jailed as a political prisoner.
The activist escaped to the UK in 2020 and has been granted asylum. He and other activists were nominated by US lawmakers for a Nobel prize in 2018.
Law told reporters at a book launch presentation in London that under Chinese rule, freedom of expression, assembly and the rule of law were "eroded in such incredible speed".
He argued that the West should pay heed as the Chinese Communist Party expands its reach around the world amid "a global decline of democracy" and "rise of authoritarianism".
"For the people living in democracy, they have to be vigilant about the political situation and to defend freedom by themselves," he told AFP.
"You can see from Hong Kong's case how rapidly it could be deteriorated."
China and the UK agreed in 1984 that Hong Kong would keep its way of life for 50 years after the UK gave up its far eastern colonial outpost in 1997.
But China has caused international concern, particularly by introducing a draconian national security law, which critics say is designed to silence dissent.
London has led Western criticism, angering Beijing and straining diplomatic ties, particularly after it effectively offered asylum to thousands of Hong Kongers.
Law said Chinese authorities grew in confidence and realised that they did not need the financial hub "to be a display window of their eagerness to catch up the West".
The 221-page book draws parallels between repressive regimes in China, Russia and Turkey.
Law argues that leaders of such regimes "do not seek revolution, but rather to guide societies towards accepting their oppression".
"We lose our freedoms when we no longer truly believe in them," he wrote.
After going into exile, Law publicly severed ties with his family in Hong Kong to protect them from reprisals.
He said the Chinese government has branded him a "national enemy" and he sees return only as a distant prospect, "maybe in decades when it's free and democratic".
"If I were to come back now, I think decades' imprisonment is the minimal charge, if not life."