Police in England and Wales are being given greater stop and search powers to tackle rising knife crime.
Home Secretary Sajid Javid is making it easier for officers to search people without reasonable suspicion in places where serious violence may occur.
But campaigners said the move was "disappointing and regressive" and that stop and search is not effective.
Stop and search powers have been controversial for many years, with evidence that they are frequently misused and that they target black people disproportionately.
But Mr Javid said: "The police are on the front line in the battle against serious violence and it's vital we give them the right tools to do their jobs."
The change is being trialled in seven police force areas where more than 60% of knife crime occurs: London, the West Midlands, Merseyside, South Yorkshire, West Yorkshire, South Wales and Greater Manchester.
It makes it easier to use so-called "section 60" checks, where for a limited period of time officers can search anyone in a certain area to prevent violent crime.
Under the new rules, inspectors will be able to authorise the use of section 60. Currently, more senior officers have to give approval.
There will also be a lower threshold. Police will only need to reasonably believe serious violence "may" occur, not that it "will".
Shadow home secretary Diane Abbott said evidence-based stop and search was "a very important tool for police".
But she added: "Random stop and search is not effective in bringing down levels of knife crime."
Section 60 has been used at large events such as Notting Hill Carnival last year and after violent incidents such as the stabbing of a man outside Clapham Common Underground station on Friday.
Other powers which account for the majority of searches will remain the same, and will still require officers to have reasonable suspicion of an offence.
With 285 deaths from stabbings in 2017-18, the most ever recorded in the UK, ministers have come under increasing pressure to tackle knife crime.
Prime Minister Theresa May will host a summit on serious youth violence on Monday.
Metropolitan Police Commissioner Cressida Dick said officers in London had increased the use of section 60 over the past 18 months, following 132 deaths from stabbings in the capital during 2017-18.
She said: "Stop and search is an extremely important power for the police. It is undoubtedly a part of our increasing results suppressing levels of violence and knife crime."
But Katrina Ffrench, chief executive of StopWatch, which campaigns against excessive use of stop and search, said: "This decision is a disappointing and regressive move, which is about politics not saving lives."
Removing the need for reasonable suspicion "will not only exacerbate the racial disparity, but has the potential to further damage the relationship between the black community and the police," she said.
Garvin Snell, an anti-knife crime activist in Hounslow, west London, said that when stop and search was "used in the correct manner", there was "nothing wrong with it".
But he added: "I grew up in an era in the 1990s when you almost felt being young and black was enough to be stopped and searched and I don't want to go back to that environment."
He said some of the extra £100m the government has promised to help reduce knife crime should be used to open more youth centres.
"A lot of these incidents are happening in poorer parts of London," he said. "Why don't we do something to raise the aspirations of these young people?"
A data study for the College of Policing into a decade of London stop and searches found them to be "inconsistent" and "weak" as a deterrent.
The extra powers reverse a key change made by Mrs May in 2014 as home secretary.
She introduced a revised code of conduct after an inquiry examined thousands of police searches and found 27% may have been illegal.
When misused, stop and search was "an enormous waste of police time" and "an unacceptable affront to justice", she said.
Reflecting on the recent announcement, the prime minister said the powers were "an important tool in the fight against knife crime".
John Apter, chairman of the Police Federation, welcomed the government's renewed support for stop and search, saying it "had been lacking for far too long".
He said it was a useful and accountable tool for officers to use in tackling knife crime and there was "no credible alternative".
Partly as a result of the 2014 changes, the use of stop and search fell in England and Wales from a peak of 1.4m ten years ago to 277,378 last year.
The numbers of searches fell for every ethnic group, but ethnic and racial inequality has grown.
In 2014-15 black people were four times more likely to be searched than white people, while in 2017-18, they were 9.5 times as likely to be targeted.