Secretary of State Mike Pompeo on Thursday threatened US sanctions over Hong Kong authorities' mass arrests of pro-democracy figures and voiced anger that an American was among those rounded up.

Pompeo said that the 53 people arrested a day earlier "should be released immediately and unconditionally."

"The United States will not stand idly by while the people of Hong Kong suffer under Communist oppression," Pompeo said in a late-night statement after a day of political violence in Washington.

"The United States will consider sanctions and other restrictions on any and all individuals and entities involved in executing this assault on the Hong Kong people," he said.

Pompeo, who remains in office for two weeks, said the United States would also "explore restrictions" against the financial hub's trade representative office in Washington.

In an operation involving 1,000 officers, Hong Kong police arrested the activists for "subversion" in an escalation of Beijing's imposition of a tough security law in the wake of huge and sometimes violent democracy protests in 2019.

Pompeo, a vehement critic of Beijing, confirmed the arrest of an American and said he was "appalled."

"Let me be clear: The United States will not tolerate the arbitrary detention or harassment of US citizens."

Two sources told AFP that the arrested American was John Clancey, a lawyer working for a firm known to take up human rights cases.

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