Tory ministers are considering the sentencing shake-up as an alternative to whole-life jail terms, under which offenders are told by a trial judge they will never be released.
The European Court of Human Rights ruled last year that whole-life sentences breached the European Convention on Human Rights because they did not include the possibility of a “right to review”.
One option being considered by the UK government is to allow judges to impose jail terms of hundreds of years, which would potentially allow offenders to have their sentences reviewed and reduced.
Mr Cameron said: “There are some people who commit such dreadful crimes that they should be sent to prison.
“Life should mean life and whatever the European court has said, we must put in place arrangements to make sure that can continue.”
However, SNP ministers have said they will not use Holyrood’s devolved powers over justice to examine a similar move.
The Scottish Government backs a new system of sentencing guidelines and a sentencing council to develop that system.
However, a Scottish Government spokeswoman said ministers would not allow courts to impose sentences of hundreds of years.
She said: “Scotland does not have automatic whole-life tariff sentences. Courts in Scotland have the discretion to set the minimum punishment period for those who commit very serious offences. In very serious cases a court could, if it wished, choose to set the minimum punishment period, before which an individual would be eligible to be considered for parole, beyond the individual’s natural lifespan.”
The spokeswoman added: “Sentencing is a matter for the independent judiciary and there are no current plans to introduce changes to these arrangements in Scotland.”
Prison sentences of 100 years or more are part of the criminal justice system in the United States and have been handed out by judges in high-profile cases.
But a leading Scottish human rights lawyer accused Mr Cameron of “posturing” on the issue and said dangerous criminals serving life sentences would not be released under existing rules.
John Scott, QC, chairman of the Howard League for Penal Reform in Scotland, said: “It’s an appalling display of thoughtless bravado. It’s pure politics and is all about votes at the next election. It’s the wrong thing to do and displays a complete lack of leadership. It’s also completely unrealistic, as if the UK government thinks it can continue to defy Strasbourg, it will give comfort to oppressive regimes wanting to do the same.
“In Scotland, people go to prison for long periods anyway and people who are dangerous would not be released by a parole board.”
Scottish Tory justice spokeswoman Margaret Mitchell suggested whole-life jail terms would be better than 100-year sentences, but said she sympathised with Mr Cameron’s stance on the issue.
She said: “The idea that life should mean life, there’s a lot of sense in that, as some people have committed crimes that are so atrocious you can’t imagine they would ever be fit to be released. Whatever you say has got to be meaningful and life should mean life if a crime is sufficiently horrendous.”
Former first minister Henry McLeish, who chaired the Scottish Prisons Commission to examine the purpose of imprisonment, attacked Mr Cameron’s plans and the idea of 100-year sentences as “madness”.
He said: “Thankfully, we don’t have such madness here.
“If you suggest that someone will go away for 100 years, there’s no incentive for the prisoner.
“Experience also shows from America that these type of sentences do nothing to reduce crime or keep people safer.”