At a Vatican meeting on Thursday with representatives from International Association of Criminal Law, the Pope said that he finds it hard to believe that countries should not have any measures other than capital punishment to defend their people.
"All Christians and people of goodwill are called to fight for the abolition of death penalty, be it legal or illegal," the pontiff said.
He mentioned the need to "improve prison conditions with respect to the human dignity of those who have been deprived of freedom. I link this to the death sentence. In the Penal Code of the Vatican, the sanction of a life sentence is no more. A lifetime in prison is a hidden death sentence".
Pope Francis added that cases where a person is given the death penalty might be subject to judicial error, and could be used by totalitarian and dictatorial regimes against opponents and minorities.
"The deplorable conditions of detention in different parts of the world are authentically inhuman and degrading, often caused by deficiencies of the criminal law, or by a lack of infrastructures and good planning."
"In many cases," he said, "they are the result of an arbitrary and merciless exercise of power over persons deprived of freedom."
The Pope cited studies by organisations defending human rights which said that the absence of motive, lack of communications and not seeing other people causes mental and physical problems. Minors and the elderly should not serve prison sentences, he added.
Pope Francis also brought up the subject of human trafficking.