Hundreds of Hong Kong protesters have stormed the headquarters of the city’s legislative council, spray-painting graffiti and smashing pictures.

The development followed an hours-long standoff that saw the building’s reinforced glass windows smashed with metal trolleys and poles, in a culmination of growing anger over an extradition bill.

The council, a mini-parliament, issued a red alert, ordering the protesters to leave immediately.

It did not say what would happen if they did not, though police did not immediately intervene.

An annual protest march was already scheduled for Monday, as the territory marks the 1 July 1997 anniversary of the handover to China from British rule.

In a statement, police said 13 officers “experienced difficulty in breathing and had swollen and itchy skin” after having an unspecified liquid thrown on them during an earlier clash where they reportedly baton-charged protesters.

The violence forced an alteration to the 1 July march, an annual tradition that shows defiance towards what critics say is the encroachment of Hong Kong’s autonomy by mainland China.

But clashes started particularly early on Monday and became unusually heated amid a backdrop of anger against city leader Carrie Lam’s attempts to force through an unpopular extradition bill. More than a million people have taken to the streets at times over the past three weeks to vent their anger.

Turbulent protests continue in Hong Kong: In pictures Show all 40 1 /40 Turbulent protests continue in Hong Kong: In pictures Turbulent protests continue in Hong Kong: In pictures An anti-extradition bill protester pour water on tear gas as they attend a march in Hong Kong Reuters Turbulent protests continue in Hong Kong: In pictures Police restrain an angry passenger who tried to fight with protesters who interrupted subway services during the morning commute in Hong Kong on July 30 AP Turbulent protests continue in Hong Kong: In pictures Protesters face off with police during the latest in a series of demonstrations against the controversial extradition bill in Hong Kong on July 28 AFP/Getty Turbulent protests continue in Hong Kong: In pictures Protesters are seen in a stairwell after one is injured by police during the latest in a series of demonstrations against the controversial extradition bill in Hong Kong on July 28 Getty Turbulent protests continue in Hong Kong: In pictures Riot police fire tear gas at protesters during the latest in a series of demonstrations against the controversial extradition bill in Hong Kong on July 28 EPA Turbulent protests continue in Hong Kong: In pictures Two police officers are surrounded by protesters on a subway platform in Hong Kong on July 30 AP Turbulent protests continue in Hong Kong: In pictures Police officers form a barricade during the latest in a series of demonstrations against the controversial extradition bill in Hong Kong on July 28 Reuters Turbulent protests continue in Hong Kong: In pictures Protesters employ the help of road signs to form a barricade during the latest in a series of demonstrations against the controversial extradition bill in Hong Kong on July 28 Reuters Turbulent protests continue in Hong Kong: In pictures A medic is attended to after being injured by police during the latest in a series of demonstrations against the controversial extradition bill in Hong Kong on July 28 Getty Turbulent protests continue in Hong Kong: In pictures Protesters are seen wearing helmets on a subway train in Hong Kong on July 30 Reuters Turbulent protests continue in Hong Kong: In pictures Protesters smash glass doors and windows of the Legislative Council Complex in Hong Kong. Anti-extradition protesters broke into the the city's legislature and thousands more gathered to march in opposition to the city's China-backed government during the 22nd anniversary of Hong Kongs return to Chinese rule Getty Turbulent protests continue in Hong Kong: In pictures Police fire tear gas at protesters near the government headquarters AFP/Getty Turbulent protests continue in Hong Kong: In pictures A protester defaces the Hong Kong emblem AFP/Getty Turbulent protests continue in Hong Kong: In pictures Protesters try to break into the Legislative Council of Hong Kong during a protest on the 22nd anniversary of the handover of the territory to China from Britain on July 1 Reuters Turbulent protests continue in Hong Kong: In pictures Black-clad riot police fired tear gas as they charged anti-government protesters in the early hours of the morning AFP/Getty Turbulent protests continue in Hong Kong: In pictures A general view of the main chamber after protesters break into the Legislative Council building EPA Turbulent protests continue in Hong Kong: In pictures Protesters graffiti the Legislative Council Complex Getty Turbulent protests continue in Hong Kong: In pictures Police fire tear gas at protesters near the government headquarters in Hong Kong on July 2, 2019. - Riot police fired tear gas as they attempted to dislodge anti-government protesters in the early hours of the morning of July 2 in chaotic scenes in the heart of Hong Kong. (Photo by Anthony WALLACE / AFP)ANTHONY WALLACE/AFP/Getty Images ANTHONY WALLACE AFP/Getty Turbulent protests continue in Hong Kong: In pictures Inside the Legislative Council building EPA Turbulent protests continue in Hong Kong: In pictures Protesters put up a sign AFP/Getty Turbulent protests continue in Hong Kong: In pictures Members of the press take pictures of a protest banner that says 'there are no violent people, only violent governance' EPA Turbulent protests continue in Hong Kong: In pictures Protesters storm the Legislative Council building during a protest in Hong Kong on July 1 Reuters Turbulent protests continue in Hong Kong: In pictures A protester forces their way through a window into the Legislative Council building in Hong Kong on July 1 Reuters Turbulent protests continue in Hong Kong: In pictures Police officers look out from behind windows damaged by protesters in the entrance to the Legislative Council in Hong on July 1 AFP/Getty Turbulent protests continue in Hong Kong: In pictures Police officers stand guard behind the entrance to the Legislative Council in Hong Kong as protesters try to force entry. One officer holds a sign that reads: "Stop charging or we use force" EPA Turbulent protests continue in Hong Kong: In pictures Protesters storm the Legislative Council building during a protest in Hong Kong on July 1 Reuters Turbulent protests continue in Hong Kong: In pictures Protesters wield makeshift shields during a clash with police on the 22nd anniversary of the territory's handover to China from Britain on July 1 Getty Turbulent protests continue in Hong Kong: In pictures Protesters holding umbrellas face-off against riot police in Hong Kong on July 1 AP Turbulent protests continue in Hong Kong: In pictures Protesters smash a window of the Legislative Council building in Hong Kong on July 1 AFP/Getty Turbulent protests continue in Hong Kong: In pictures A protester wielding a makeshift shield looks back at the damage caused to the Legislative Council building in Hong Kong on July 1 Reuters Turbulent protests continue in Hong Kong: In pictures Protesters gather to take part in the annual pro-democracy rally that is held in Hong Kong on the anniversary of the handover of the territory from Britain to China on July 1 AFP/Getty Turbulent protests continue in Hong Kong: In pictures Police officers use force in an attempt to disperse protesters during a protest in Hong Kong on the 22nd anniversary of the handover of the territory from Britain to China on July 1 Reuters Turbulent protests continue in Hong Kong: In pictures Anti-extradition bill protesters stand behind a barricade near the Legislative Council of Hong Kong on July 1 Reuters Turbulent protests continue in Hong Kong: In pictures Police officers look out from behind windows damaged by protesters in the entrance to the Legislative Council in Hong on July 1 AFP/Getty Turbulent protests continue in Hong Kong: In pictures Protesters try to break into the Legislative Council of Hong Kong during a protest on the 22nd anniversary of the handover of the territory to China from Britain on July 1 Reuters Turbulent protests continue in Hong Kong: In pictures Protesters rest outside the Legislative Council in Hong Kong on July 1 EPA Turbulent protests continue in Hong Kong: In pictures A protester shouts in front of police outside the Legislative Council in Hong Kong on July 1 AFP/Getty Turbulent protests continue in Hong Kong: In pictures A group of anti-extradition bill protesters holding umbrellas gather outside the government headquarters in Hong Kong on July 1 AFP/Getty Turbulent protests continue in Hong Kong: In pictures A protester waves a black version of the Hong Kong flag outside of the Legislative Council of Hong Kong on July 1 Getty Turbulent protests continue in Hong Kong: In pictures Anti-extradition bill protesters march in Hong Kong on the anniversary of the territory's handover to China on July 1 Reuters

Police fought to keep back hundreds of helmeted protesters who tried to advance down closed streets towards the official harbour-front venue marking the anniversary.

Attendants were asked to be seated inside a huge convention centre rather than gather outside as they have in previous years, with the government blaming light rain for the change.

During the event, Chinese and Hong Kong flags were raised together and two helicopters and a small flotilla passed by.

In a five-minute speech, Hong Kong chief executive Ms Lam said the protests of recent weeks had taught her she needs to listen better to the youth, and to 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.

Ms Lam insisted her government had 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 the pro-democracy lawmaker Helena Wong out of the room as she walked backwards shouting at Ms Lam to resign and withdraw the “evil” extradition legislation. She later told reporters she was voicing the grievances and opinions of the protesters, who could not get into the event.

Perhaps the most dramatic scenes were at the legislative council building, however, where police in full riot gear stood lined up inside the plate glass windows as protesters tried to gain entry, repeatedly ramming and cracking the windows with anything they could find. The standoff lasted well into the sweltering evening, and the Reuters news agency reported the protesters had breached and stormed the building late on Monday.

Fearing a repeat of the heavy-handed police tactics from the bloody 12 June protest, pro-democracy politicians urged the public to demonstrate peacefully.

Opposition politicians including Lam Cheuk-ting, Hui Chi-fung, Roy Kwong Chun-yu and HK First’s Claudia Mo were at the scene at the council headquarters, attempting to discourage the crowd from violence, CNN reported.