Hong Kong pro-democracy activist Joshua Wong has been arrested in jail on a new charge, while a detained American lawyer has been granted bail.

Mr Wong, 24, was arrested on Thursday morning on suspicion of violating the controversial national security law imposed on Hong Kong last June, his friends and family said.

The leading activist, who is serving a 13-month prison sentence for organising and participating in an unauthorised protest in 2019, was taken away to give a statement on the new charge, a post on his Facebook page said.

Tam Tak-chi, a fellow activist in prison for "uttering seditious words", was arrested as well, local media reported.

On Wednesday, 53 activists and Democrat Party members were arrested over their organisation and participation in last July's unofficial primary election for the since-postponed 2020 Legislative Council election.

The arrests mark a major increase in the use of the security law - which was brought in by China last year and prompted months of protests.

It made acts of subversion, secession, terrorism and collusion with foreign powers to intervene in Hong Kong affairs illegal, with a maximum punishment of life imprisonment.

It has been strongly criticised by many Western governments, who have accused China of trying to crush any hint of dissent.

Hong Kong Police said the 53 arrested are accused of committing subversion under the law after they proposed using strategic voting to secure a legislative majority to veto government budgets.

Police said the ultimate goal of the accused was to force the territory's chief executive, Carrie Lam, to resign and to shut down the government.

One of those arrested was American human rights lawyer John Clancey, who works at law firm Ho Tse Wai and Partners.

He was granted bail on Thursday, his associate said.

Most of those arrested were candidates in the primary but Mr Clancey was a treasurer for political organisation Power for Democracy, which was involved in the event.

Former legislative council member Au Nok-hin was bailed on Wednesday as he was in quarantine for COVID-19 when he was arrested, NOW TV reported.

The rest of the group are expected to be bailed on Thursday without charges - for the moment - but most have had their passports confiscated, an unnamed source told the South China Morning Post.

UK foreign secretary Dominic Raab called the arrests a "grievous attack on Hong Kong's rights and freedoms".

He said they were protected under the Joint Declaration signed by China and the UK when Hong Kong was handed back to China in 1997.

"These arrests demonstrate that the Hong Kong and Chinese authorities deliberately misled the world about the true purpose of the National Security Law, which is being used to crush dissent and opposing political views," he said.

"The UK will not turn our backs on the people of Hong Kong and will continue to offer British Nationals (Overseas) the right to live and work in the UK."

Mike Pompeo, the US secretary of state, said he was "appalled" and called it a "campaign of political oppression", while the UN also condemned the arrests.