US President-elect Joe Biden's pick for Secretary of State blasted Hong Kong authorities for arresting some 50 opposition figures on Wednesday under a new national security law.

"The sweeping arrests of pro-democracy demonstrators are an assault on those bravely advocating for universal rights," Antony Blinken wrote on his verified Twitter account.

"The Biden-Harris administration will stand with the people of Hong Kong and against Beijing's crackdown on democracy," he added.