LONDON — Ali started carrying knives when he was 14, long before his childhood friend was stabbed to death while walking down a well-lit street in South London in March.

“When you’re leaving the house, you don’t even think about it. You grab your keys, wallet, phone and knife,” said Ali, a 19-year-old student who provided only his first name out of fear that he would be approached by the police.

Ali said he had never been involved with the gangs and violence that are associated with the Surrey Lane public housing project where he lives. Nevertheless, like many young people in London, he said he carried a knife with the idea of protecting himself.

The trend of young men arming themselves with knives has become an enormous concern for the Metropolitan Police, Britain’s largest police force, which last month reported a 24 percent increase in knife crimes over the last year, bringing knife-related offenses to a five-year high.