Hong Kong police arrested activist Joshua Wong for subversion Thursday - a day after jailing 53 people for the same crime over their participation in legislative primaries run by the city's pro-democracy parties last year.

The detainees - including 20 sitting district councilors and 14 former Legislative Council members -  are accused of subversion for their roles in an unofficial vote among Hong Kong's opposition political groups that saw candidates promise to veto all government budgets if elected.

More than half a million people cast votes in the July democratic camp's primaries, organizers said at the time. The election was to determine which candidates would run in the canceled Legislative Council election due this past September.

"Those arrested are guilty of nothing but exercising the democratic rights promised to them by treaty, and due to them through virtue of their humanity. They should be released immediately and unconditionally," U.S. Secretary of State Mike Pompeo in a statement Thursday.

Wong's house was raided Wednesday morning but no additional charges were laid until Thursday. The 24-year-old activist is currently in jail for participating in an unauthorized protest last year outside police headquarters in Wan Chai.

Also arrested Wednesday were union leaders, former professors, journalists and lawyers - including American solicitor John Clancey.

"I am appalled by the news of the arrest of an American citizen as part of this campaign of political repression," Pompeo said.

"Let me be clear, the U.S. will not tolerate the arbitrary detention or harassment of U.S. citizens," he said hinting at another round of economic sanctions in retaliation.

Clancey practices law at the firm Ho Tse Wai & Partners, founded by former Democratic Party chairperson Albert Ho. He is treasurer for the group Power for Democracy which helped organize the unofficial primaries last year.

Mainland China officials in the city welcomed the mass arrests and expressed "firm support" for the police in a statement.

"The Hong Kong police arrested today 53 individuals...who acted toward the aim of paralyzing the government. [They] have allegedly violated the crime of subversion in...the National Security Law," a representative for the China Liaison Office said.

The pro-establishment New People's Party also praised the authorities for cracking down on subversive activities - noting that the concept of primary elections is not a part of Hong Kong's political system.