The prime minister says he is concerned about crime "rebounding" as lockdown ends.
He said the government is "redoubling" efforts on knife crime and gangs and said his thoughts are with a mother whose son was stabbed to death at the weekend.
Boris Johnson told reporters on Monday: "We've seen a big fall in crime figures in the last few months but I'm worried about it rebounding as we come out of lockdown."
The comments come as Sven Badzak, 22, was killed on Saturday and his friend is fighting for his life after they were chased and attacked by a group of males in Kilburn, northwest London, at around 5.30pm.
Jasna Badzak, a former Conservative Party activist, shared pictures on Twitter of her son as a child with Boris Johnson, George Osborne, and David Cameron, alongside a plea for help.
Mr Badzak was on his way to a supermarket when he was attacked.
The prime minister said his thoughts were "very much with [Ms Badzak] and her family in mourning the loss of her son".
Ms Badzak said Sven was her only child and that he "was my everything".
"I invested all my life for him to have the best possible life," she told Sky News. "He could charm everybody, even the hardest people."
She described him as a "future prime minister", and described her final conversation with her son.
"[He said] 'love you mum', I said 'I love you too, please come back soon' - that was the last conversation I ever had with him."
He said he wanted to ensure young people do not get drawn into the "nihilistic" culture of gangs.
The prime minister said he was worried about crime figures picking up after a "big fall" during the pandemic, but that authorities were now "redoubling efforts" to clamp down on violence.
"I think that's what happening on the streets of too many of our cities is very, very sad and I want to see kids protected from some of the gang crime, the knife crime, the culture of violence that they're all too often sucked up in," said Mr Johnson.
He said efforts would focus on a "tough policing response" such as stop and search and "proper custodial sentences for young people who go equipped with knives".