Boris Johnson’s criminal justice reforms have been branded a “monumental waste of money” and “a lot of hot air”.
But his reforms have been criticised by penal reform charities and think tanks, described as "unevidenced electioneering" by one group.
"Boris Johnson's criminal justice reforms are a monumental waste of money,” said Charlotte Pickles, director of the Reform think tank.
“His proposed sentencing reforms ignore evidence that shows that longer prison sentences are ineffective at deterring crime or reducing re-offending.
"His unequivocal embrace of stop and search is dumbfounding - research shows it does little to prevent violence.
"Intelligent investment is clearly need, but these pledges are costly election baubles, not a serious attempt to make this country safer."
The prime minister said dangerous criminals must be taken off the streets and punishments "truly fit the crime" if the public was to have confidence in the justice system.
He also announced an extra £85 million for the Crown Prosecution Service (CPS) to help it manage its caseload over the next two years.
The move follows a series of announcements over the weekend in which Mr Johnson promised to "come down hard" on crime .
They included a £2.5 billion programme to create 10,000 additional prison places and the extension of enhanced stop-and-search powers to police forces across England and Wales.
It will fuel speculation that Mr Johnson is preparing the ground for an early general election amid continuing deadlock in Parliament over Brexit.
Frances Crook, chief executive of the Howard League for Penal Reform, also criticised Mr Johnson’s plans.
She said: "We have excessively long prison sentences already and it doesn't seem to be keeping us safe.
"What's coming out of Number 10 is politics but not real life. It's not going to deal with real-life crimes and victims. It's a lot of hot air.”
She said the idea “will create more crime and more victims but they may well get themselves elected”.
She added: "Community sentences reduce crime better than prison sentences.
"I have seen politicians trying to whip up a lynch mob mentality for their own benefit before and that does appear to be what's happening now."
New prisons are just as violent and cause just as much reoffending as old prisons - see Berwyn, Doncaster, Isis, etc. It’s a lie - and I use that strong word advisedly - to tell the public that building huge new jails will reduce crime— Frances Crook (@francescrook) August 11, 2019
Peter Dawson, director of the Prison Reform Trust, said: "Doing the same thing over and over again and expecting different results strikes most people as a mistake.
“We have been here so often before - most recently with Michael Gove promising and failing to build 10,000 more prison spaces in 2016.
"Announcing more prisons to make short term political capital is evidence free policy making, diverting attention from the real problems facing our prison and justice systems."
On Monday, Mr Johnson told leading figures in the criminal justice system that young people must be prevented from getting on "the conveyor belt to crime".
He said "you cannot just arrest your way out of a problem" as he addressed a group assembled in No 10, including the most senior police officer, Cressida Dick.
Mr Johnson said "faster justice" was required and cited pledges including increasing prison capacity and employing more officers.
"But no matter what we do with the criminal justice system we also have to recognise that you cannot just arrest your way out of a problem," he added. "And I think all police officers, all representatives of the criminal justice system, will know that.
"You have to address the whole problem and, number one, you've got to stop young people becoming criminals, stop them getting on what used to be called the conveyor belt to crime, turn their lives around earlier, give them opportunities, hope and encouragement that they need."
Home secretary Priti Patel was among the group alongside Metropolitan Police Commissioner Ms Dick invited into the State Dining Room in Downing Street, as was Sir Brian Leveson, who previously held the title of the most senior criminal judge in England and Wales, and Solicitor General Michael Ellis QC.
Mr Johnson said the alleged attempted murder of a police officer in Birmingham over the weekend was further evidence of "the threats that they face, the risks that the police run to keep us safe".
He stressed that "the police need our backing", as he said he would ensure they have "the powers that they need", in particular stop and search.
The sentencing review has been instructed to start work immediately and to report back to No 10 in the autumn, just as the country may be going to the polls.
Its remit is to look at the rules governing how and when violent and sexual offenders are released from prison.
Currently, offenders sentenced to 12 months or more serve the first half of their time in prison and the second "on licence" in the community, where they may be subject to recall.
The review will consider whether changes to legislation are needed so that more time is spent in jail.
Mr Johnson will highlight the approach at a Downing Street round-table bringing together leaders from the police, probation and prison sectors on Monday.
He said: "Dangerous criminals must be kept off our streets, serving the sentences they deserve - victims want to see it, the public want to see it and I want to see it.
"To ensure confidence in the system, the punishment must truly fit the crime. We have all seen examples of rapists and murderers let out too soon or people offending again as soon as they're released.
"This ends now. We want them caught, locked up, punished and properly rehabilitated."
However, critics warned that there was no evidence that longer sentences would result in a reduction in crime.
Liberal Democrat justice spokeswoman Wera Hobhouse said the UK already had the largest prison population of any country in Western Europe and that a different approach was needed.
"For years, Labour and Tory ministers have made sentences longer and longer, without any evidence that they prevent crime," she said.
"It may sound tough, but it hasn't made our communities any safer. All it does is overcrowd our prisons and waste millions of pounds.
"We do need a sentencing review, but its aim should be to reverse decades of pointless sentence inflation. Instead of just talking tough, it's time ministers look at the evidence."
The Solicitor General, Michael Ellis QC, said the additional £85 million for the CPS would ensure it was equipped to deal with an increase in cases brought to them by the police.
Chris Henley QC, chair of the Criminal Bar Association, added: "The criminal justice system is severely underfunded, as a result of relentless cuts over the last 10 years.
"Any change to that direction of travel is to be welcomed. The CPS is desperately in need of significant investment to improve its capacity to deliver effectively for the public, but this has to be seen as a modest first step in the rebuilding process.
"Far more will be needed across the system including more substantial sums to the entire prosecution system and the Ministry of Justice including the courts service to restore public faith in our criminal justice system where increasingly those who commit crime walk free and the innocent risk being convicted."