China on Monday sanctioned 11 Americans, including senators Marco Rubio and Ted Cruz, in retaliation for similar US moves prompted by Beijing's crackdown in Hong Kong.

Washington last week accused 11 officials of suppressing "freedom and democratic processes" in Hong Kong, including city leader Carrie Lam, and announced plans to freeze their US assets.

It was the toughest US action yet in response to Beijing's introduction of a sweeping and controversial new national security law for the territory.

Beijing said the measure was a violation of international law and "grossly interferes in China's internal affairs".

"China has decided to impose sanctions on some people that behaved badly on Hong Kong-related issues," foreign ministry spokesman Zhao Lijian said Monday, with Human Rights Watch director Kenneth Roth and National Endowment for Democracy president Carl Gershman also on the list.

Zhao did not give details of what the sanctions would entail.

Republican senators Rubio and Cruz established themselves as two of the most vocal supporters of Hong Kong's democracy movement last year, when the city was convulsed by huge and sometimes violent protests.

Beijing has accused "external forces" of fomenting unrest and responded to the unrest by imposing the security law in late June, sending a political chill through the semi-autonomous finance hub.

Since then, the city's leaders have postponed local elections, citing the coronavirus pandemic.

Authorities have also issued arrest warrants for six exiled pro-democracy activists and launched a crackdown on other activists.

On Monday, Hong Kong media mogul Jimmy Lai, one of the city's most vocal Beijing critics, was arrested under the security law.

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