A 30-year-old protester nicknamed "second generation Captain America" has been detained for questioning in Tseung Kwan O police station following his arrest for breaching the national security law.

The suspect, Ma, a deliveryman who often dressed as the Marvel Comics character at protests, was arrested for "inciting and abetting others to commit secession" under the new law on Tuesday.

Shortly before 6pm Tuesday, he was said to have been heard chanting "Liberate Hong Kong, revolution of our times" during an assembly that marked the one-year anniversary of the death of then 15-year-old Chan Yin-lam.

The slogan, coined by localists including Edward Leung Tin-kei, is in breach of the new law, as the government deemed it "pro-independence, secessionist and subversive."

Ma was also the administrator of a Telegram protest group named "610,000 people are not in fear of being arrested," which he started last month. The group forwarded social media posts by Leung and the Hong Kong National Party - an outlawed localist party.

People online urged those who have joined the Telegram group to exit the group or delete any politically sensitive conversations.

David Li Kwok-wing, known as "lunch brother" due to his frequent appearances at various lunchtime rallies, said on his Facebook that whether Ma will be released on bail soon is unclear, adding that he may have to attend trials immediately.

A total of 27 people - 21 men and six women - have been arrested for engaging in acts and activities endangering national security since the law came into effect on July 1, police said.

These include 19-year-old former Studentlocalism convener Tony Chung Hon-lam and three former group members aged between 16 and 21.

The Civil Human Rights Front said yesterday that it has applied for a letter of no objection from the police to organize a rally on October 1.

They plan to march from Causeway Bay to Central, repeating protesters' five demands and urging the mainland to release the 12 detained Hongkongers.

Police are prepared to deploy at least 2,500 officers to any protest organized for National Day. The protest was suggested by members of a Telegram group named "October 1 Remembrance Day march," and will call for police accountability for the shooting of a then 18-year-old student in October. The protest will also oppose the launch of a health code aimed at allowing quarantine-free travel and the national security law.

'Captain america' caught in law net