VATICAN CITY — Pope Francis said Thursday that keeping inmates isolated in maximum security prisons is “a form of torture,” and called life sentences “a hidden death penalty” that should be abolished along with capital punishment.

“All Christians and people of good will are called today to struggle not only for abolition of the death penalty, whether legal or illegal, and in all its forms, but also to improve prison conditions, out of respect for the human dignity of persons deprived of their liberty,” the pope told delegates from the International Association of Penal Law.

“And this I connect with life imprisonment,” he continued. “Life imprisonment is a hidden death penalty.”

The pope noted that the Vatican recently eliminated life imprisonment from its own penal code, though that move was largely symbolic.

In the wide-ranging address, Francis denounced practices that are widespread in many regions of the world, such as extrajudicial executions and detentions without trial, which he said account for more than half of all detentions in some countries.

Francis also denounced corruption in penal systems, calling it “an evil greater than sin.”

The corrupt, he said, are like people with bad breath: They don’t realize they have it and must be told by others. Yet corruption has seeped into every corner of society, from business to public works contracts to national security operations. But the penal system operates like a net that “catches only the little fish while leaving the big fish free to swim the ocean.”

Those corrupt “big fish,” he said, are the ones who should be punished most severely because they damage all of society.

Francis also took aim at practices that have been hotly debated in the United States, such as the so-called “extraordinary rendition” of terror suspects to other countries, which the pope described as the practice of “illegal transportation to detention centers in which torture is practiced.”

The pope called out both the nations that use such practices and those who allow it to happen on their territory or allow the use of their air space for other countries to transport detainees.

In addition, Francis said isolation in so-called “supermax” prisons, which are sometimes used for convicted terrorists or the most dangerous criminals, can be “a form of torture.” That’s because such treatment can lead to “psychic and physical sufferings such as paranoia, anxiety, depression, and weight loss and a significantly increased chance of suicide.”

Francis said that rather than focusing so much on punishment as the means to redress every social ill — “penal populism,” he called it — nations should pursue broader policies of “economic and social inclusion.”

Previous popes, notably Pope St. John Paul II, were outspoken opponents of capital punishment, and Francis on Thursday repeated the words of his predecessor and the teaching of the catechism that says that cases justifying execution “are very rare, if not practically nonexistent.”

“It is impossible to imagine that states today cannot make use of another means than capital punishment to defend peoples’ lives from an unjust aggressor,” Francis said.

A Gallup poll released Thursday showed that support for the death penalty in the US remains steady, with about 63 percent backing capital punishment for those convicted of murder and 33 percent opposed.