"We need the people of Hong Kong to respect the law. So if a piece of legislation has been enacted but people refuse to abide by the law, then of course we have a problem at hand," she said.

She said 10 per cent of protesters arrested on Sunday were aged under 15 years.

Hong Kong police said on Tuesday that 77 people, the youngest aged 12, had been arrested under the mask ban, which came into effect on Saturday.

Among them, 74 people were arrested for “using facial covering at an unlawful assembly”. Three were arrested for failing to remove their mask in a public place when requested by police.

The education department asked school principals to report how many students wore masks on Tuesday, but one principal told local media that school grounds weren't covered by the public order law so the ban shouldn't apply.

US President Donald Trump said on Monday that trade talks with China would be affected if anything "bad" happened in Hong Kong and he wanted anti-government protests to be handled humanely.

China's commerce ministry announced a high-powered trade delegation, including vice premier Liu He, People's Bank of China governor Yi Gang, and commerce minister Zhong Shan, would arrive in Washington on Thursday.

A deadline of October 15 looms for the next round of US tariffs on Chinese goods.

The Chinese delegation includes at least four vice-ministers for finance, technology and agriculture and indicates a willingness by China to strike a deal and end the trade war.

Lam told media at her regular press briefing: "I still strongly feel that we should find the solution ourselves.

"That's also the position of the central government, that Hong Kong should tackle the problem on our own. But if the situation becomes so bad, then no option should be ruled out if we want Hong Kong to at least have another chance."