Hong Kong police arrested prominent democracy activist Joshua Wong on Thursday for participating in an unauthorised assembly in October 2019 and violating the city's anti-mask law, according to a post on his official Twitter account.

Wong's latest arrest adds to several unlawful assembly charges or suspected offences he and other activists are facing related to last year's pro-democracy protests, which prompted Beijing to impose a sweeping national security law on June 30.

Hong Kong police confirmed they arrested two men, aged 23 and 74, on Thursday for illegal assembly on Oct 5, 2019.

The arrest of Wong, aged 23, comes around 6 weeks after media tycoon Jimmy Lai was detained on suspicion of colluding with foreign forces.

Wong had been a frequent visitor to Washington where he appealed to the U.S. Congress to support Hong Kong's democracy movement and counter Beijing's tightening grip over the global financial hub. His visits drew the wrath of Beijing, which says he is a "black hand" of foreign forces.

Wong disbanded his pro-democracy group Demosisto in June, just hours after China's parliament passed national security law for Hong Kong, bypassing the city's local legislature, a move widely criticised by Western governments.

His long-time colleague, Agnes Chow, and two other activists were also among 10 people police arrested in August on suspicion of violating the new law.

The new law punishes anything China considers as subversion, secession, terrorism and collusion with foreign forces, with up to life in prison.

Wong was just 17 years old when he became the face of the 2014 student-led Umbrella Movement democracy protests, but he was not a leading figure of the often violent unrest that shook the semi-autonomous former British colony last year.

An anti-mask law was introduced last year in a bid to help police identify the protesters they suspected of committing crimes and it is facing a challenge in court. In the meantime, the Hong Kong government has made face masks mandatory in most circumstances due to the coronavirus pandemic.

The European Union on Thursday criticised the arrest of the prominent activist, saying it was "troubling" and undermined trust in China.

"The arrest of Hong Kong pro-democracy activist Joshua Wong on 24 September is the latest in a troubling series of arrests of pro-democracy activists since the summer," an EU spokesperson said, calling for "very careful scrutiny" by the judiciary.

"Developments in Hong Kong call into question China's will to uphold its international commitments, undermine trust and impact EU-China relations."

The EU has repeatedly voiced concern at the new Hong Kong security law, which critics say erodes important freedoms in the city.

Last week senior EU leaders pressed Chinese President Xi Jinping over the situation in Hong Kong at a video summit, saying democratic rights in the city must be preserved.