Labour leader Jeremy Corbyn has attacked the Conservatives over police cuts amid rising violent crime, as he launched his party's London election campaign.
"You simply cannot maintain community cohesion when you slash funding to the police service," he said.
More than 50 people have been killed in violent attacks in London since the start of the year.
Ministers say falling police numbers are not to blame for rising violence.
However, a leaked Home Office document seen by the BBC has concluded a drop in officers "likely contributed" to a rise in serious violent crime.
The paper was drawn up by officials looking at a new strategy to tackle violence in England and Wales that is being set out later by Home Secretary Amber Rudd.
Voters go to the polls across England on 3 May for local elections with seats on about 150 councils up for grabs. London will be one of the key electoral battlegrounds, with every borough council seat up for election.
Mr Corbyn said his election launch comes in "the shadow of one of the most violent periods we have experienced in recent times - not just here in London but across our country".
Political leaders, "have to tackle full-on the reasons so many of our young people are having their lives extinguished", he said, promising to meet families of victims, police and campaigners to shape Labour's response.
"You simply cannot maintain community cohesion when you slash funding to the police service and cut the number of officers on our streets by 21,000," he added.
"You cannot protect local communities when you cut funding to local councils to such an extent they are unable to provide the essential youth service support that stops many young people from being drawn into violent crime.
"The Tory record on policing and crime is one of reckless failure. Plain and simple - you can't have security on the cheap and cuts have consequences. Too many communities are living with those consequences."
The Conservatives say police are being given the resources they need to fight violent crime, and that Labour's handling of the economy would mean it could not do this.
Mr Corbyn also said the Grenfell Tower fire, which killed 71 people last year, highlights inequality in British society, saying it "simply would not have happened if the occupants had been wealthy".
More than 50 people have been killed in violent attacks in London since the start of the year - three teenagers were killed in separate incidents last week: Tanesha Melborne, 17, Amaan Shakoor, 16, and Israel Ogunsola, 18.
Home Secretary Amber Rudd wrote on Sunday that police numbers were rising in the early 2000s - when serious violent crimes were at their highest.
The government's new strategy to tackle serious violence includes an Offensive Weapons Bill for England and Wales, with new rules on the carrying of knives and acid.
It will target the so-called county lines drug distribution model where city gangs courier heroin and crack cocaine to rural or coastal towns, and will call on social media companies to do more to remove violent gang content.