The Hong Kong government said Wednesday it had cancelled the passports of six democracy activists who fled to the United Kingdom, calling them "lawless wanted criminals".

Hong Kong has intensified a crackdown on dissent since authorities quashed massive, at times violent, pro-democracy protests in 2019, enacting security laws that critics such as Britain and the United States say have curbed its unique freedoms.

The city issued HK$1 million ($128,000) bounties last year for 13 activists based abroad who authorities accused of committing national security crimes.

The six identified Wednesday -- all on the bounty list -- are considered "lawless wanted criminals... hiding in the United Kingdom", a government spokesperson said in a statement.

"They continue to blatantly engage in activities that endanger national security. They also make scaremongering remarks to smear and slander the Hong Kong Special Administrative Region."

Besides cancelling their Hong Kong passports, police said anyone offering funds, leasing property or running a business with them could face up to seven years in jail.

The six are former lawmaker Nathan Law, veteran unionist Christopher Mung Siu-tat, and activists Finn Lau, Fok Ka-chi, Choi Ming-da and Simon Cheng.

Hong Kong officials cited a national security law passed in March, colloquially known as Article 23, as the legal basis for cancelling their passports.

Security chief Chris Tang said the measures were "necessary" because the six were "harboured in the United Kingdom and continue to collude with foreign forces".

Asked if people would violate the law by subscribing to the activists' content on online platforms such as Patreon and YouTube, Tang said "it is an offence to provide funds or to handle funds for those specified absconders, no matter what platform it is".

Lau said he had only ever held a British National Overseas passport, which is available to Hong Kongers born in the former British colony before the 1997 handover to China.

"It is ridiculous to cancel (a Hong Kong passport) that never exists," he said on X, adding it "does not deter me from advocating human rights & democracy".

Mung called the passport cancellations "shameless political retaliation".

"The regime can cancel my passport but it can never revoke my identity as a Hong Konger... One day I will get back my Hong Kong passport from a liberal, democratic system," Mung told AFP.

Cheng, who founded the civil society group Hongkongers in Britain, said the measures had little effect on his daily life but condemned the government for trying to "isolate us from our supporters and allies".

China's foreign ministry spokesman Lin Jian blasted the six, saying their "nasty behaviour seriously endangered national security, seriously damaged the fundamental interests of Hong Kong, and seriously attacked the bottom line of 'one country, two systems'."

- Bounties issued -

Under the Sino-British agreement for the handover, Hong Kong enjoys more freedoms and rights than its mainland counterparts and once had a robust opposition bloc that advocated for more democratic processes.

Wednesday's move came on the fifth anniversary of a violent clash between protesters and police that marked a major escalation in the 2019 pro-democracy protests.

After quashing the protests, Beijing imposed a sweeping security law on Hong Kong in 2020 which critics say has broken down the legal firewall that once existed between the city and mainland China.

The law, under which nearly 300 have been arrested, also claims the power to hold accused people accountable across the world.

Article 23, the homegrown security law passed in March, granted Hong Kong authorities further enforcement powers.

The six identified Wednesday have been accused of security crimes, including incitement to secession, incitement to subversion and foreign collusion, that could land them in prison for life.

City leader John Lee, who is under US sanctions for his role as security chief in 2019, said last year they would be "pursued for life".

Five others in Hong Kong have been arrested for allegedly providing financial support for the wanted activists. They were later released on bail.

Around 40 family members and former colleagues have also been taken in for police questioning over the past year.