Combative protesters tried to break into the Hong Kong legislature on Monday as a crowd of thousands prepared to start a march in that direction on the 22nd anniversary of the former British colony's return to China.

With a crowd of a hundred or so people around them, a small group of people repeatedly rammed a cargo cart into a glass panel of the building, wedging the cart partially through the damaged safety glass.

The unexpected disruption stalled the start of the march on Monday. The crowd has started filing out of Victoria Park but police asked the marchers to change their route or cancel the march.

Both the combative protesters and the marchers oppose a Government attempt to change extradition laws to allow suspects to be sent to China to face trial. The proposal has increased fears of eroding freedoms in the territory that was returned to China in 1997.

The embattled leader of Hong Kong pledged to be more responsive to public sentiment in a speech at a flag-raising ceremony.

Carrie Lam has come under withering criticism for trying to push through the legislation. She said a series of protests and marches that have attracted hundreds of thousands of students and other participants in recent weeks have taught her that she needs to listen better to the youth and people in general.

"This has made me fully realise that I, as a politician, have to remind myself all the time of the need to grasp public sentiments accurately," she said in a five-minute speech to the gathering in the city's cavernous convention center.

She insisted her Government has good intentions, but said "I will learn the lesson and ensure that the government's future work will be closer and more responsive to the aspirations, sentiments and opinions of the community."

Security guards pushed pro-democracy lawmaker Helena Wong out of the room as she walked backward shouting at Lam to resign and withdraw the "evil" legislation. She later told reporters she was voicing the grievances and opinions of the protesters, who could not get into the event.

The annual march starting in the afternoon was expected to be larger than usual because the proposed extradition bill has awakened broader fears that China is eroding the freedoms and rights guaranteed to Hong Kong for 50 years under a "one country, two systems" framework.

Two marches in June against the legislation drew more than a million people, according to organiser estimates.