The US and British governments have condemned the arrests of 15 pro-democracy activists in Hong Kong, describing them as "inconsistent" with China's international commitments.

Key points: One serving and nine former legislators were arrested, including veteran activists

One serving and nine former legislators were arrested, including veteran activists The arrests were made on charges of organising and participating in protests

The arrests were made on charges of organising and participating in protests The raids mark the biggest crackdown on the pro-democracy movement since anti-government protests in June

"The United States condemns the arrest of pro-democracy advocates in Hong Kong," US Secretary of State Mike Pompeo said.

US Attorney-General William Barr also weighed in with a statement saying these events show how "antithetical the values of the Chinese Communist Party are to those we share in Western liberal democracies.

"These actions — along with its malign influence activity and industrial espionage here in the United States — demonstrate once again that the Chinese Communist Party cannot be trusted."

Britain's Foreign Office also criticised the arrests, saying, "the right to peaceful protest is fundamental to Hong Kong's way of life and as such is protected in both the Joint Declaration and the Basic Law".

Beijing has accused the US and other Western countries of instigating the protests and insists they are China's internal affairs.

The Office of the Commissioner of the Chinese Foreign Ministry in Hong Kong said police were enforcing the law against those suspected of organising and participating in unauthorised assemblies, and foreign countries have no right to interfere, China's official Xinhua News Agency reported.

"It is completely wrong that the UK Foreign Office spokesperson has distorted the truth by painting unauthorised assemblies as 'peaceful protests,' in a bid to whitewash, condone and exonerate the anti-China troublemakers in Hong Kong," the statement said.

Among those detained on charges of illegal assembly were Democratic Party founder Martin Lee, 81, millionaire publishing tycoon Jimmy Lai, 71, and former politician and barrister Margaret Ng, 72, according to media and political sources.

Police said they were all arrested for assembling unlawfully on three different days last year during unauthorised protests that police had not approved.

China has accused Western powers of interfering in its internal affairs. ( AP: Frank Augstein )

Veteran activists Lee Cheuk-yan and Yeung Sum were also arrested.

Democratic legislator Claudia Mo, who was not among those detained, said the city Government, led by Chief Executive Carrie Lam, was trying "to introduce a ring of terror in Hong Kong".

"They are doing whatever they can to try to silence, to take down, the local opposition," Ms Mo said, pointing to upcoming legislative elections in September in which democrats hope to win back veto power in the city assembly.

The group was due to appear in court on May 18, but police superintendent Lam Wing-ho said more arrests were possible. Some of those arrested were later released on bail.

New push for security law

Mass protests targeting a now-scrapped bill proposing to send suspects to mainland China for trial erupted last year. ( Reuters: Tyrone Siu )

Mass protests first erupted in June last year, initially targeting a now-scrapped bill proposing to send suspects to mainland China for trial.

The protests broadened into demands for full democracy and a public investigation into the use of force by police.

After his release on bail on Saturday afternoon (local time), Martin Lee said he did not regret his actions.

"I'm proud to have the chance to walk our democracy road with Hong Kong's excellent young people," he said.

Authorities in Hong Kong have arrested more than 7,800 people over their involvement in the protests, including many on rioting charges that can carry jail terms of up to 10 years.

It is not clear how many of them remain in custody.

Saturday's arrests come after several months of relative calm amid a partial coronavirus lockdown but as Chinese and city government officials launch a new push for tougher national security laws for the city.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 42 seconds 42 s Police and students have engaged in street battles throughout the city.

Hong Kong returned to Beijing in 1997 under a "one country, two systems" formula that guaranteed broad freedoms not seen in mainland China, and a high degree of autonomy.

A previous attempt to draft a national security law for Hong Kong, known as Article 23, was met with mass protests in 2003 and abandoned.

Hong Kong Government and security officials have recently described some of the democracy movement's actions as being close to terrorism.

ABC/Reuters