Millions have taken to the streets in the past three months to protest against an extradition bill that would have allowed people to be sent to mainland China for trial in courts controlled by the Communist Party.

The protests are the most serious political crisis in Hong Kong since it returned to China 22 years ago. They also pose the greatest popular challenge to Chinese leader Xi Jinping since he came to power in 2012 and come as Xi already grapples with a Sino-U.S. trade war and tensions in the South China Sea.

Some activists have waved the U.S. flag during the demonstrations, while Beijing has accused U.S. officials of being behind the violent chaos and warned against interference.

Trump was asked by reporters before leaving for a campaign event in Ohio on Thursday whether he was concerned by media reports that China might intervene in Hong Kong and said the city had experienced "riots for a long period of time".

"Hong Kong is a part of China, they'll have to deal with that themselves," he said.

Trump's labelling of the demonstrations as riots is certain to rile activists in the Asian financial centre who have called on the city's government to drop the use of the word to describe the protests.

On Wednesday, 44 people were charged with rioting for their role in one recent protest that turned violent when thousands of activists clashed with police near Beijing's main representative office in the city.

Rioting is defined under Hong Kong law as an unlawful assembly of three or more people where any person commits a breach of the peace. A conviction can carry a 10-year prison sentence.

Trump said earlier this month he believed Xi had acted  responsibly with the Hong Kong protests and that "China could stop them if they wanted".

Thousands of civil servants are planning to rally on Friday night in support of the protesters and to urge authorities to rebuild confidence in the government, with more demonstrations planned for the weekend.

What started out as protests against the extradition bill have evolved into a broader backlash against the Hong Kong government and its political master in Beijing.