Time stood still in Parliament Square today as a mother cried out for her son.

We all felt it. The loss of another Black man. The raw, collective outpouring of grief in the community has grown more profound with each life that has been taken at the hands of police.

Heads hung, the crowd thought about Chris Kaba, the 24-year-old unarmed Black man who was shot dead by a firearms officer in Streatham, south London, on Monday night. We thought about the laughs he’ll never get to have, the soon-to-be-born baby he’ll never get to raise, the life he’ll never get to live.

The Audi Chris was driving was rammed before being boxed in by officers during a police chase. He was killed by a single shot that entered through the driver’s side of the car’s windscreen.

Police watchdog the Independent Office for Police Conduct has confirmed that no gun was found in the vehicle.

Protesters led by Chris’s family marched along Whitehall towards Scotland Yard on Saturday, a day after the IOPC announced its homicide investigation.

But the watchdog has warned: “It doesn’t necessarily mean that [the officer] will end up facing criminal charges or a misconduct hearing.”

The family is calling for the immediate suspension of the officer who fired the shot.

Addressing the hundreds of demonstrators who turned up to show their support, family spokesperson and Chris’s cousin Jefferson Bosela said: “When I found out the tragic news that Chris was killed I was at work… I cried on the floor, absolutely inconsolable. It felt like a nightmare and I knew my world was about to be turned upside down.

“One word that remained at the forefront of my mind, one word that I kept repeating to myself… is why? We deserve the opportunity to look his killers in the eye and ask why.

“Why did you not have mercy on him as he sat there in the car, defenceless, unarmed, scared?... Why did you destroy the possibility for me to tell him that I love him, and why did you destroy the possibility for him to reply? Why does a mother have to grow old while her first born child is six feet under?”

Lee Jasper, chief steward at the demonstration, told openDemocracy: “Time is up.”

Speaking about new Metropolitan Police chief Mark Rowley, he said: “This new commissioner is coming in, and on his very first day, on 14 September, when he appears at the GLA committee, we will be there waiting for him and making sure that he gets the message that we’ve only just started.”

The new commissioner and former counter-terror chief is set to take up his new post while the force is in special measures.

“We’ve secured the victory of forcing the IOPC to open a homicide investigation… I hope we can take this case all the way and make Chris Kaba the last Black man to die [at the hands of police],” Jasper added.

Delia Mattis from Black Lives Matter Enfield said: “It was an assassination. An unarmed man was murdered… He was sat in his car, they boxed him in and they assassinated him.

“The fight is a hard fight, and it takes a long time. It’s not going to be one protest that does it… The establishment is used to this. They see us doing a protest and they think [we’re] going to get tired soon. You’ve got to commit to demanding justice.”

Deborah Coles, Director of INQUEST, a charity that provides expertise on state-related deaths, warned of false information by the police. “When the state kills… it produces false narratives. They don’t give us enough information, or they give false information. They try and demonise those who die to try and justify their lethal force. We know this isn’t just about state violence, and brutality and neglect, but it’s about the racism and misogyny of state institutions…

“Yet again, it’s families who have to force the organisations responsible to tell the truth - something that should be their legal and moral right... Without the family’s legal support, the political and community support… this homicide investigation would not have been started.”

Parallels have been drawn with the killing of Mark Duggan, who in 2011 was shot dead by a Met officer in Tottenham. Speaker Temi Mwale of youth empowerment organisation 4Front Project said: “If [police] listened after Mark, Chris would be alive.”

She added: “I see sisters… mothers… aunties. I see the mandem… brothers… uncles… grandfathers.”

Generations, from toddlers to grandparents, had gathered to show support for Chris’s family. As Mwale pointed out, young people in the crowd, many in their 20s and 30s, once sat on their own parents' shoulders at protests as children, marching for the same thing.

Chris joins a long line of Black men who have been killed by police officers. As the IOPC investigation continues, speakers including rapper Stormzy have urged protesters to “have stamina” in the fight for justice.

Ending her address to the crowd, Mwale spoke directly to the young Black men present, saying: “All too often our young men are demonised. I can see that brothers from all different ends are here.

“I see you… I know what it meant for you to come here today, because these people [police] are harassing you, targeting you, brutalising you. This is about you. This is for you.”