Thousands of people have taken part in two massive marches to try to stop young people carrying knives.
Families who've lost relatives to knife crime and their supporters chanted "Stop the knives, save lives" as they walked through the streets of London.
So far this year, 45 teenagers have been victims of knife crime in Britain.
Two women started the idea for the People's March on the internet, after the death of 16-year-old Ben Kinsella, who was stabbed in London in June.
A separate anti-knife demonstration is being held in Scotland
The two marches met in Hyde Park where celebrities, sports stars and politicians joined a rally to make a stand against violence.
Richard Taylor, whose son Damilola died in 2000 when he was stabbed, said they wanted to send a message to young people that knife killings must stop.
"There are other ways by which you can sort out a fight rather than use a knife," he said.
Elsewhere, about 2,000 people took part in an anti-knife demonstration in Inverdale in Scotland.