A group of protesters in Hong Kong has swarmed into the legislature building after breaking windows and prying open metal security bars in a fresh wave of demonstrations that have plunged the Chinese-ruled city into political turmoil.

Key points: Tensions arose ahead of a flag-raising ceremony to mark the handover to China

Tensions arose ahead of a flag-raising ceremony to mark the handover to China Riot police swooped on protesters who had blockaded a street near the venue

Riot police swooped on protesters who had blockaded a street near the venue Hong Kong was returned from British to Chinese rule on July 1, 1997

Police appeared to back-off as the protesters came in to avoid confrontation.

Pro-government politicians have complained of radical, violent elements undermining social order.

The protesters said the Hong Kong government is not responding to their demands for a full withdrawal of contentious extradition legislation and the resignation of city leader Carrie Lam.

The renewed chaos in the Asian financial hub came after police and protesters faced off earlier in the day.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 7 seconds 1 m 7 s Hong Kong protestors attempt to storm government HQ

Police used pepper spray and batons against anti-government protesters who had seized highways ahead of what was expected to be a huge pro-democracy rally on the anniversary of the city's handover to China.

The semi-autonomous city has been shaken by huge demonstrations in the past three weeks as protesters demanded the withdrawal of a bill that would allow extraditions to the Chinese mainland.

Tensions spiked once more in the financial hub as dawn broke on Monday after thousands of mainly young, masked protesters seized three key thoroughfares, deploying metal and plastic barriers to block the roads.

Shortly before a flag-raising ceremony to mark the handover, police swooped on protesters who had blockaded one street, with at least one female protester seen bleeding from a head wound after the clashes.

The ongoing rallies are the latest manifestation of growing fears that China is stamping down on the city's freedoms and culture with the help of the financial hub's pro-Beijing leaders.

A police officer strikes an anti-government protester during the recent protest. ( Reuters: Thomas Peter )

The British returned Hong Kong to Chinese rule on July 1, 1997, but it is still administered separately under an arrangement known as "one country, two systems".

Many residents fear Beijing is already reneging on that deal.

Pro-democracy activists have organised a march every handover anniversary, calling for greater democratic freedoms, such as the right to elect the city's leader.

They've mustered large crowds in recent years — including a two-month occupation in 2014 — but failed to win any concessions from Beijing.

A group of mostly young protesters tried to block access to a symbolically important ceremony marking the anniversary. ( AP: Kin Cheung )

This year's rally comes against the backdrop of unprecedented anti-government protests over the past three weeks that have drawn millions of people and anger over police using tear gas and rubber bullets to clear crowds.

The spark for the current wave of protests was an attempt by chief executive Carrie Lam to pass the Beijing-backed extradition law, which she has now postponed following the huge public backlash.

But the demonstrations have morphed into a wider movement against Ms Lam's administration and Beijing, posing the greatest popular challenge to Chinese leader Xi Jinping since he came to power in 2012.

A tired-looking Ms Lam appeared in public for the first time in nearly two weeks to attend the anniversary ceremony, flanked by her husband and former Hong Kong leader Tung Chee-hwa.

Authorities deployed a huge security blanket around the venue, with the ceremony moved inside shortly before it was due to begin.

Ms Lam said the government needed to change its style of governance to be more accommodating and open and pledged to do more for young people.

The anniversary of Hong Kong's handover to China has always attracted pro-democracy rallies but this year's event will be especially charged. ( Reuters: Tyrone Siu )

"I, as a politician, have to remind myself all the time of the need to grasp public sentiments accurately," she said.

"After this incident, 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."

The turmoil comes at a delicate time for Beijing, which is grappling with a trade dispute with the United States, a faltering economy and tensions in the South China Sea.

Protesters holding umbrellas face off against riot police on a Hong Kong street. ( AP: Kin Cheung )

AFP/Reuters