Scottish Labour leader Iain Gray has said 500 prison places are needed to cope with his party's policy of six-month jail terms for knife-carriers.
Mr Gray said the policy was workable despite more than 3,000 people being convicted of the offence last year.
Tory leader Annabel Goldie said the courts needed discretion on sentencing but the Lib Dems' Tavish Scott said jail terms would not cut re-offending.
SNP leader Alex Salmond said knife crime fell 30% in the past four years.
The four main party leaders clashed as they went head-to-head on BBC Scotland's Politics Show, in the run-up to the 5 May Scottish Parliament elections.
During talks on crime, Mr Gray said he wanted mandatory sentences for anyone convicted of carrying a knife, in a policy designed to "deter".
His party plans to lock up for six months anyone convicted of carrying a knife.
Figures from the Scottish government showed 3,194 crimes of having a knife in public were committed in 2009-10, a drop of 22% on the previous year.
"Currently, of those who are convicted of carrying a knife, 70% don't go to jail. Only 30% do," he said.
Mr Gray said 500 additional prison places would be needed but £20m had been put aside in the manifesto to make the policy "good".
He added: "This is a policy which has come from the experience of the victims of knife crime and their families and those who fear for their safety on the streets."
Miss Goldie argued anyone convicted of a crime should be serving the sentence imposed and not being released early.
She added: "The court must have discretions in sentencing.
"There may be a world of difference between someone innocently carrying a knife going to a neighbour's allotment to cut vegetables and someone with a knife at two in the morning in a city centre."
Scottish Lib Dem leader Mr Scott added: "Senior chief constables, senior people who I respect and listen to on this matter, say the policy that Labour and the Tories have had will add to the prison population, won't cut crime and will just mean reoffending goes on and on and on - that's not the answer to a problem we all know we've got to solve."
Mr Salmond said the reduction in knife crime was boosted by initiatives such as violence reduction units and police officers working at the front line.
He added: "When Iain was the deputy justice minister, knife crime increased in a single year by 12%.
"People want not just politicians addressing the issue but addressing the issue on the basis of the facts."
In what was a confrontational debate, they also agreed action was needed on alcohol, to tackle Scotland's so-called "booze-and-blades" culture.
Mr Gray went on to say he would be prepared to discuss minimum alcohol pricing, the SNP plan rejected by MSPs in the last parliament.
He said: "We will talk about it - the proposal has to be workable."
Labour, the Lib Dems and Tories rejected the proposal, saying it penalised responsible drinkers and was "probably" illegal, under European competition law.
Mr Gray said the SNP must be prepared to listen to alternatives, such as caffeine limits on drinks such as Buckfast tonic wine.
Miss Goldie added: "The proposal has to target the problem drinks and the problem drinkers, not penalise people."
Existing legislation, Mr Scott said, was not being properly used.
"There have only been three prosecutions under the previous licensing law passed by parliament which were specifically meant to deal with these and other growing problems of alcohol abuse in Scotland," he said.
Mr Salmond said the minimum pricing proposals did not infringe competition law and said it would not be possible to introduce legislation to parliament if it had been deemed illegal.
He added: "We'll be introducing minimum pricing for alcohol and I do hope we can get the support that the doctors and police and society in Scotland demand from their politicians."
When asked what qualities each leader admired about their opponent, Mr Scott said: "I'm not interested in personalities, I'm much more interested in policies."
Miss Goldie said she admired Mr Salmond's ability to "deliver Conservative policies", while Mr Gray said: "Alex is a showman and an actor and I will never be those things."
Mr Salmond said: "I hugely admire the forbearance of all these three excellent estimable people in putting up with these other rotters and their political parties."