A peaceful rally in Hong Kong has descended into chaos as police fired teargas and water cannon at protesters who hurled petrol bombs, set fires and clashed with residents.

Tens of thousands of demonstrators defied a police ban and marched on the seat of the government calling for greater democracy in the semi-autonomous Chinese territory.

Hundreds of protesters who splintered off from the march surrounded the government complex, throwing rocks and molotov cocktails, prompting riot police to respond with multiple rounds of teargas as well as water cannon that covered demonstrators in blue dye, meant to help identify protesters for arrest later.

Facebook Twitter Pinterest Police outside the British consulate-general on Sunday. Photograph: Vivek Prakash/EPA

In a statement, the police said they deployed teargas, rubber bullets, sponge rounds and “crowd management vehicles” on demonstrators participating in an “unauthorised assembly”. It also said two police officers withdrew pistols at one point when they were attacked by around 20 protesters who hurled petrol bombs at them.

The Hong Kong government issued a late night statement “severely condemning the violent acts which totally disregard law and order.”

Skirmishes broke out in various locations across Hong Kong as protesters scattered after the police dispersal. In the residential area of Fortress Hill, men in white T-shirts beat people with rods and stools. Lam, a 30-year-old man who asked to only give his surname, was knocked to the ground and kicked repeatedly by a group of men.

“I saw men rushing out of [metro] station with chairs to hit … they were doing the beating. I yelled, ‘someone is beating [people] and they rushed me … and beat me,” he said.

Quick guide Democracy under fire in Hong Kong since 1997 Show Hide Hong Kong’s democratic struggles since 1997 1 July 1997: Hong Kong, previously a British colony, is returned to China under the framework of “one country, two systems”. The “Basic Law” constitution guarantees to protect, for the next 50 years, the democratic institutions that make Hong Kong distinct from Communist-ruled mainland China. 2003: Hong Kong’s leaders introduce legislation that would forbid acts of treason and subversion against the Chinese government. The bill resembles laws used to charge dissidents on the mainland. An estimated half a million people turn out to protest against the bill. As a result of the backlash, further action on the proposal is halted. 2007: The Basic Law stated that the ultimate aim was for Hong Kong’s voters to achieve a complete democracy, but China decides in 2007 that universal suffrage in elections for the chief executive cannot be implemented until 2017. Some lawmakers are chosen by business and trade groups, while others are elected by vote. In a bid to accelerate a decision on universal suffrage, five lawmakers resign. But this act is followed by the adoption of the Beijing-backed electoral changes, which expand the chief executive’s selection committee and add more seats for lawmakers elected by direct vote. The legislation divides Hong Kong's pro-democracy camp, as some support the reforms while others say they will only delay full democracy while reinforcing a structure that favors Beijing. 2014: The Chinese government introduces a bill allowing Hong Kong residents to vote for their leader in 2017, but with one major caveat: the candidates must be approved by Beijing. Pro-democracy lawmakers are incensed by the bill, which they call an example of “fake universal suffrage” and “fake democracy”. The move triggers a massive protest as crowds occupy some of Hong Kong’s most crowded districts for 70 days. In June 2015, Hong Kong legislators formally reject the bill, and electoral reform stalls. The current chief executive, Carrie Lam, widely seen as the Chinese Communist party’s favoured candidate, is hand-picked in 2017 by a 1,200-person committee dominated by pro-Beijing elites. 2019: Lam pushes amendments to extradition laws that would allow people to be sent to mainland China to face charges. The proposed legislation triggers a huge protest, with organisers putting the turnout at 1 million, and a standoff that forces the legislature to postpone debate on the bills. After weeks of protest, often meeting with violent reprisals from the Hong Kong police, Lam announced that she would withdraw the bill. Photograph: Dale de la Rey/AFP

Fights also broke out in the nearby neighbourhood of North Point, a pro-government district where dozens of local residents jeered at riot police who arrived at the scene to break up the fights. Pro-democracy lawmaker Ted Hui was arrested late night for obstructing police after he tried to persuade them to release a couple, reported public broadcaster RTHK. A university student reporter was also taken away by police for having a knife in his bag, even though he claimed it was for cutting moon cakes for the Mid-Autumn Festival the night before, RTHK said.

Sunday’s clashes came after ugly scenes of brawls between anti-government and pro-Beijing demonstrators on Saturday, marking the 15th consecutive weekend of mass protests that have roiled Hong Kong for the last three months.

Much of the demonstrations are centred on public anger toward the police as authorities crack down harder on protesters by force. More than 1,000 people have been arrested.

Facebook Twitter Pinterest Pro-democracy protesters react as police fire water cannon outside the government headquarters. Photograph: Isaac Lawrence/AFP/Getty Images

“Please tell the world,” said Lo, 41, who lives in Fortress Hill, where bystanders yelled at the police to leave. “We are suffering. We are scared every day when the police come out.”

The demonstrators, many of them young students, have also become more violent. On Sunday, they set fires outside a metro station and smashed glass windows and display cases inside the stations. Video footage posted online showed what appeared to be a group of black-clad demonstrators mobbing a man. In another, a group was seen beating up a young man.

Crowds of demonstrators had marched earlier in the day, chanting “Resist Beijing, liberate Hong Kong”. Many held up their right hands in reference to the five demands of the protesters, which include an independent investigation into police behaviour.

Quick guide What are the five demands of the Hong Kong protesters? Show Hide The complete withdrawal of the proposed extradition bill Hong Kong’s leader, Carrie Lam, has said her government will formally withdraw the bill that ignited months of protests. Hong Kong residents had feared it could be used by China to extradite people for political reasons. They want guarantees that it cannot be reintroduced at a later date. Withdrawal of the use of the word 'riot' in relation to the protests Protesters want the government to officially recognise that their movement has been a series of legitimate protests, rather than a riot, as has been stated in official communications. Unconditional release of arrested protesters and charges against them dropped Hundreds of people have been arrested in recent weeks, and the protesters are demanding that all of them be freed, and that no convictions should stand against any of them. An independent inquiry into police behaviour Police use of force has escalated since the demonstrations began, while protesters have also resorted to increasingly violent measures. Demonstrators say an inquiry into police brutality is the number-one priority. Implementation of genuine universal suffrage Hong Kong's chief executive is currently selected by a 1,200-member committee, and nearly half of the 70 legislative council seats are filled by limited electorates representing different sectors of the economy. The protesters want to be able to vote for their leaders in free and open democratic elections. Photograph: Anushree Fadnavis/X06783

The protests mark Hong Kong’s – as well as Beijing’s – most serious political crisis in decades, triggered by a proposal to allow extradition to mainland China. As the protests have dragged on, confrontations between police and protesters have become increasingly violent.

“If we fail this time, we will not have a second chance. Our civil society will be repressed by the Chinese Communist party. So we have to keep going,” said Gerald Chan, 24, a master’s student who was wearing reflective goggles and held a shield improvised from street signs.

Facebook Twitter Pinterest A protester catches fire after a molotov cocktail was thrown during a demonstration near the government complex in Hong Kong. Photograph: Tyrone Siu/Reuters

Hong Kong’s leader, Carrie Lam, has said she will permanently withdraw the controversial bill but the demonstrations have expanded to take on new demands, including an independent investigation into the police and implementation of universal suffrage.

“This is not the Hong Kong we know,” said Tim Cheng, 42, a father of two, who said the demand he most wanted to see met was the inquiry into police behaviour. “When I was young, my parents and teachers told me that if I was in trouble to contact the police. Now I can’t tell my kids that.”

Earlier on Sunday, hundreds of pro-democracy protesters had gathered outside the British consulate in Hong Kong chanting “we will not surrender” as they called on the UK to come to the aid of the former British colony, handed over to Chinese control in 1997.

Demonstrators waved the union flag, sang God Save the Queen and chanted “Stand for freedom, stand with Hong Kong”.

China has called the handover agreement signed by the UK and China in 1997 – the joint declaration – a “historical document” with “no practical significance”.

Facebook Twitter Pinterest An activist holds a petition letter addressed to the consul general as pro-democracy demonstrators gather outside the British consulate-general building. Photograph: Nicolas Asfouri/AFP/Getty Images

Chinese officials have lashed out at the UK for signalling its support of Hong Kong. A UK Foreign Office spokeswoman in June said the document was a “legally binding treaty … that remains as valid today as it was when it was signed”.

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Beijing and pro-China figures in Hong Kong have previously accused foreign powers including the US and the UK of secretly funding and organising the rallies.

On Saturday, Joshua Wong, the democracy activist and former student leader of the 2014 umbrella movement in Hong Kong, called on US officials to include Hong Kong and a human rights clause in ongoing negotiations as the US and China try to resolve a long-running trade war and for lawmakers to pass legislation that could sanction Hong Kong officials.

On Sunday, families, middle-aged and elderly residents, marched, chanting “Reclaim Hong Kong!” and held signs that said “Guard the next generation”.

“We owe them a lot, they have sacrificed a lot. For us, just going out to the protests is the minimum we can do,” said Des, 50, a demonstrator who often comes to protests with his 11-year-old son.

“This is our last chance. You can see the momentum. People are still coming out. We need to take this one last chance.”

Additional reporting by Verna Yu