Thousands of anti-racism protesters have gathered across the UK, defying calls from ministers and police chiefs to avoid mass gatherings. Crowds of mostly young people on Saturday heard speeches in Parliament Square, London, declaring the beginning of the end of institutional racism, and observed a minute’s silence on one knee to commemorate black people killed by police in the UK. “Institutional racism is rooted in housing, healthcare, the education system, the media, in fashion and beauty,” Imarn Ayton, 29, told the crowd through a megaphone. “It is rooted in recruitment and employment, in politics, in policing, in immigration, and the criminal justice system. “It is seen but you cannot see it. It is everywhere but it is nowhere … Today is the day where we say goodbye to institutional racism.” However, as the day wore on, tensions began to flare after a white man made racist remarks, prompting a person to throw a drink at him. He was led away by police following a tense standoff in which police pushed protesters back after one officer was hit in the face with a plastic bottle. Soon after, police guarding Downing Street were jeered, taunted and pelted with glass bottles and flares, leading riot police to replace them and officers on horseback advanced on protesters at about 6pm amid sudden downpours of rain. One officer was taken to hospital with non life-threatening injuries after her horse panicked and hit a traffic light as the unrest grew. Police were reinforced, with more than 150 officers standing off with a crowd of up to 500 who chanted choruses of “Boris Johnson is a racist” and also accused police of racism as they were kettled. Further flashpoints erupted and officers brawled with protesters, with at least 14 arrests in total throughout the day and another nine officers injured, as glass bottles and flares continued to be thrown, while dozens of protesters danced to music played from a powerful speaker before some joined in prayer. Many opposed violence and discouraged the throwing of projectiles, but numerous others were determined to oppose officers and those remaining were reportedly kettled into Sunday, with legal observers reporting protesters had to say their names if they wanted to leave. The mayor of London, Sadiq Khan, praised those who protested peacefully but said the “tiny majority” who became violent undermined the cause. “To the thousands of Londoners who protested peacefully today, I stand with you and I share your anger and your pain. George Floyd’s brutal killing must be a catalyst for change worldwide,” he said. “To the tiny minority who were violent and threw glass bottles and lit flares – you endangered a safe and peaceful protest and let down this important cause.” Earlier in the day, demonstrators had splintered, with some marching to the Home Office, led by a number of drummers, to demand justice for the Windrush generation. A larger group including Madonna and Rio Ferdinand remained in Westminster, while others converged on the US embassy, with physical distancing mostly impossible because of the depth of the crowds throughout the capital. Holding banners reading “there is a virus greater than Covid-19 and it’s called racism” and “the UK is not innocent”, protesters called for education at an early age about colonialism and an end to the disproportionate use of stop and search as the drizzle cleared and a festival atmosphere permeated the crowds. “Police mentality needs to change, they need to be educated that this is not okay, stop and search is a prime example of how racist the UK is,” said a 24-year-old demonstrator who preferred to remain anonymous. “They target black people, they target minorities, they know what they’re doing … And we need to teach children how the UK was funded by racism, [just like] America.” Alex Kouam, 29, from west London, led chants calling for justice as a group of protesters passed Buckingham Palace. “Evidently the UK is not innocent,” he said. “The Lammy report highlighted a number of discrepancies in the criminal justice system relating to black and minority ethnic people. But of course the government hasn’t acted upon it. This is a clear indication of institutional racism in the UK.” A couple of officers had knelt before protesters in an act of solidarity on Wednesday, but the Metropolitan police commissioner, Cressida Dick, publicly warned told officers not to do so again and they seemingly abided by the order. Supt Jo Edwards announced late on Saturday that a post-event investigation would be carried out following the unrest. “We understand people’s passion to come and let their voice be heard, they protested largely without incident,” she said. “Our officers have been professional and very restrained but there was a smaller group intent on violence towards police officers. “Twenty-three officers have received injuries, doing their job, policing protest over the last few days, and that is totally unacceptable.” Elsewhere, about 15,000 people were estimated to have demonstrated in Manchester, while thousands more attended protests in Cardiff, Glasgow, Leicester, Milton Keynes and Sheffield, and in many other towns and cities in the UK to demand an end to systemic racism and in solidarity with George Floyd, the black man who died after a white police officer knelt on his neck in Minneapolis almost a fortnight ago. Chanting, clapping in unison and holding homemade placards saying “black lives matter”, a crowd filled Piccadilly Square in Manchester to listen to speakers. They fell silent at 1.45pm and knelt to pay their respects to Floyd. The world heavyweight champion boxer Anthony Joshua joined the protest in his hometown of Watford. “We can no longer sit back and remain silent on this senseless, unlawful killings and sly racism on another human being – based on what? Only their skin colour,” the 30-year-old said, standing with crutches. “We need to speak out in peaceful demonstrations – just like today, so, well done, Watford. We must not use a demonstration for selfish motives and turn it into rioting and looting.” Several hundred marchers gathered in Newcastle while thousands more watched an online protest organised in the north-east. Dr Christina Mobley, a history lecturer who came to Newcastle University from the University of Virginia in Charlottesville, attended with her five-year-old daughter. The historian, who is leading a project to decolonise the university curriculum, said: “I absolutely felt the need to be here today. The organisers have done an amazing job. It is really powerful to see such a young, motivated crowd coming out and organising themselves, handing out masks and working with the police.” In Belfast and Londonderry, police said organisers of protests attended by a total of about 1,000 people would be reported to the Public Prosecution Service “with a view to prosecution” for violating coronavirus lockdown regulations – a decision that could cause controversy. “The health protection regulations are in place to protect us all during this pandemic and it is everyone’s responsibility to adhere to them to protect our society,” said assistant chief constable Alan Todd. The home secretary, Priti Patel, said on Saturday that although she understood the right to protest, the UK was in the midst of a pandemic. “I’d say to those that want to protest: please don’t,” she said, echoing the health secretary, Matt Hancock. “The regulations are very clear in terms of gatherings and mass gatherings in particular. We must put public health first at this particular time.”