The claim: Half of the violence in London is attributable to street gangs, as well as half of knife crime and 60% of gun crime.
Reality Check verdict: There isn't much public data available on gang crime. Nearly half of reported knife crime in London in 2016-17 was committed against young people and not by a family member, according to the Mayor's Office for Policing and Crime.
But that doesn't necessarily mean it was attributable to street gangs. According to the Metropolitan Police, which looks at whether a crime involved an identified gang member, 0.19% of violent crime was linked to gangs. The definition of a gang is contested.
Former Conservative leader Iain Duncan Smith said that half of all violent crime in London was carried out by gangs, as the think tank he chairs, the Centre for Social Justice, launched a report on the problem.
There is no national data on gang crime but the Mayor's Office for Policing and Crime (Mopac) in London produced estimates for the city up until December 2017. The Centre for Social Justice's report used these estimates.
There were 4,446 reported offences of knife crime with injury in total in the year to March 2017, according to the Metropolitan Police.
And Mopac took the number of these in which the victim was under the age of 25 and excluded incidents of domestic violence.
This left 2,028 incidents. That's 45% of the total.
But we don't know that all of these incidents were gang crime - only that they involved young victims and that the perpetrator wasn't a family member.
Mopac has since replaced this measurement with a system called the "weapon-enabled crime dashboard" and no longer attempts to put a figure on gang-related crime.
And even using this measurement, the proportion of gun crime put down to gangs in the CSJ report doesn't come close to 60%. Mopac's estimate - using the same methodology as for knife crime - found that only 14% of gun crime was gang-related.
The Centre for Social Justice said its much higher figure came from Home Office research from 2006, in which 80 young men aged 18-30 convicted of firearms offences were interviewed. About half said they had been in a "gang or crew".
The Metropolitan Police also "tags" crimes as gang-related if it believes it has intelligence to support this.
The force holds a database of "persons of interest", known as the Gang Matrix, and has stated that known gang members were responsible for 24% of stabbings where the victim was under 25, and 7% of all knife offences, in 2016.
Last year, it said less than 1% of all violent crime in the capital was tagged in its crime recording system as being related to gangs.
However, the database has been the subject of controversy after it was accused of disproportionately targeting black men who might not have links to violent crime.