Tony Chung was previously arrested on suspicion of breaching the new national security law and had been released on bail. He was detained by plain-clothes policemen at a coffee shop near the consulate on Tuesday.
Two other members of Mr Chung’s now-disbanded activism group Studentlocalism, Yanni Ho and William Chan, were also arrested.
The police, while not confirming the identity of the suspects, said two men and a woman, aged between 17 and 21, were in custody and an investigation was underway.
Studentlocalism identified those taken in custody as former convenor Mr Chung and former members Mr Chan and Ms Ho. “Currently, Tony Chung is still detained at Central Police District Headquarters, it is still unknown whether he can be released on bail or not,” organisers of the disbanded group said.
Mr Chung was taken away from a coffee shop close to the US consulate by unidentified men, South China Morning Post first reported, adding that he was believed to be preparing to make an asylum application.
The UK-based Friends of Hong Kong said Mr Chung had gone to the US consulate in the city at 8.10 am but it did not open for another 20 minutes. He called the organisation from a coffee shop less than 50 metres from the consulate, and said he had been followed by a man in a green shirt who he feared was a national security agent. “About four minutes later we received reports from a passerby that Tony was snatched away from Pacific Coffee and led towards [a nearby government] building,” the spokesman told the Guardian.
Hours later, Ms Ho and Mr Chan were also arrested when they reported to a police station in connection with their previous arrest on 29 July, according to the Facebook page of Studentlocalism.
The US consulate has so far not issued a statement on the matter, but an SCMP report said four other activists who did enter the consulate’s grounds to seek asylum were turned away.
Mr Chung was the first public figure arrested under the controversial new national security law in July on suspicion of being involved in an organisation that vowed to fight for an independent Hong Kong. He had been free on bail after the initial arrest.
Like other pro-democracy organisations, Studentlocalism disbanded before Beijing imposed the national security law.
The former British colony of Hong Kong returned to Chinese rule in 1997 and has been rocked by pro-democracy demonstrations in recent years.
Beijing imposed the national security law on 30 June, in a move widely condemned by Western governments and human rights groups. The legislation outlaws secessionist, subversive and terrorist acts, as well as collusion with foreign forces in the city's internal affairs. The maximum punishment for serious offenders is life imprisonment.
Others who have been arrested under the legislation include media tycoon Jimmy Lai, who founded the pro-democracy tabloid Apple Daily, and prominent pro-democracy activist Agnes Chow.
So far, 28 people have been arrested under the new legislation, according to a police post on Facebook.
China considers Hong Kong to be an “inalienable” part of the country, and calls for independence are anathema to Beijing’s Communist Party leaders.