British Prime Minister Sir Keir Starmer says he "felt sick" while watching a video released by police of an 18-year-old student who was handcuffed while he lay dying from stab wounds after his killer falsely alleged a racist attack.Henry Nowak died after the knife attack in the southern England city of Southampton last December.Warning: This article contains graphic content.Mr Nowak died after being stabbed stabbed with a knife his attacker, Vickrum Digwa, said he was permitted to carry due to exemptions for Sikhs to have ceremonial daggers. (Reuters: Isabel Infantes)His killer, Vickrum Digwa, a 23-year-old Sikh man, was sentenced to life in prison on Monday, local time, having lied to police at the time that Mr Nowak had assaulted him.In police body cam footage, Mr Nowak is seen lying on his back on the street, saying "I've been stabbed" and "I can't breathe" while an officer responds, "I don't think you have, mate."Sir Keir said the video proved that "there are clearly serious questions that need to be addressed"."Henry Nowak was a kind, thoughtful, and much-loved young man, and this has understandably been devastating for his parents, for his family, and everybody who knew him," he said."My thoughts, I'm sure the thoughts of everybody in the country, are with Henry's family. His life has been stolen, and they are understandably devastated."I have seen the body cam footage. It's harrowing, and I have to say, as a father of a 17-year-old boy, I felt sick watching it."It is absolutely right that the IOPC (Independent Office for Police Conduct) is looking at this. There are clearly serious questions that need to be addressed, not least how accusations of racism informed the decision-making in this case," he said.Mr Nowak was filmed on police body cam telling responding officers that he had been stabbed and he couldn't breathe. (AP: Gareth Fuller / PA)Death sparks heated protests in SouthamptonA large group of more than 1,000 protesters marched through Southampton's city centre towards the scene of the crime on Tuesday, local time, and attacked police blocking the road with stones, bottles and bins.Vickrum Digwa was sentenced to life in prison over Mr Nowak's death. (AFP: Supplied / Hampshire & Isle of Wight Constabulary)Some members of the rally who gathered outside the city's main police station were heard chanting "two-tier scum" and "shame on you!" while waving British Union Jack and England flags, AFP reported.Far-right leader Tommy Robinson told the crowd that "if Henry (Nowak) weren't white, he wouldn't have been handcuffed" and that "as white people we are treated as second-rate citizens by our own police force".Protesters, some wearing masks, then marched to the residential area where the crime took place and attacked a line of police wearing helmets and carrying riot shields, chanting "scum".American tech tycoon Elon Musk has posted on X an offer to fund a private prosecution against the police over their handling of the murder.Protesters faced off with lines of police in Southampton during a heated rally. (Reuters: Isabel Infantes)UK Home Secretary Shabana Mahmood condemned the behaviour of some protesters, labelling it "completely unacceptable"."The Nowak family made a powerful call to us all yesterday to not let Henry's death be used to create further division, hatred or tension," she said in a statement posted on X."There can be no justification for hijacking this tragedy to stir up violence and disorder."Those responsible can expect to face the full force of the law."Tommy Robinson (centre) was among the crowd of protesters in Southampton. (AP: Gareth Fuller / PA)Video reveals teen's 'inhumane and degrading treatment'Digwa stabbed Mr Nowak with a knife he said he was permitted to carry due to exemptions for Sikhs to have ceremonial daggers.When the police arrived, Digwa said his turban had been knocked off, and he had an injury to his eye.Mr Nowak's family called his treatment by police "inhumane and degrading", but, in a statement outside court, his father said his death should not be "used to create further division, hatred or tension".Henry Nowak's father, Mark, says he hopes his son's death does not spark further division. (AP: Will Heaver / PA)That was echoed by Ms Mahmood, who told parliament on Tuesday, local time, that everyone was equal before the law and urged calm during the investigation."We must condemn those who seek personal political profit from tragedy," she said.She also warned that threats against police and inflammatory commentary were worsening the situation, and that public services should assess only the risk a person poses, not race or religion.Some police officers were targeted by smoking flares thrown towards them by protesters. (Reuters: Isabel Infantes)Referencing previous efforts to tackle racism in policing, Ms Mahmood said: "Whatever changes are made, it is important that nobody over-corrects or course-corrects such that all of us as citizens are no longer equal before the law."She called for calm as the IOPC investigates the conduct of the officers from the Hampshire and Isle of Wight Constabulary and said that rumours spread online had led to death threats against an officer who wasn't involved in the arrest."Misinformation and inflammatory commentary are making a dreadful situation even worse," she said."We must all together condemn it."Nigel Farage, whose anti-immigration Reform party leads opinion polls, said the incident was an example of the rights of ethnic minorities trumping those of white British people.He said people should respond to the incident with "pure cold rage," and called for an end to "anti-white prejudice" and the promotion of the idea "that white lives matter just as much as black lives".He sought to draw parallels with the 2020 killing of George Floyd in the US, which sparked the Black Lives Matter movement.Mr Floyd had also said "I can't breathe" as a police officer kneeled on his neck for several minutes.The Hampshire Police have apologised for Mr Nowak's death.In a separate statement on Tuesday, the force said one of the officers involved in the arrest had resigned, while three others were being treated as witnesses in the investigation.Some protesters also attacked police officers controlling the crowds by throwing bricks, stone, bottles and bins. (Reuters: Isabel Infantes)Mr Nowak's death comes two years after a separate stabbing rampage that killed three girls and wounded 10 other people at a dance class in northern England, which led to nearly a week of widespread rioting after people incorrectly identified the teen suspect on social media as a Muslim asylum-seeker.The fiery and violent clashes with police were mostly aimed at migrants and Muslims.The parents of the British-born attacker in that case were Christians from Rwanda, and investigators haven't been able to pin down his motivation, but they ruled out terrorism.Police found documents about subjects including Nazi Germany, the Rwandan genocide and car bombs on his devices.Wires