Francis made the comments during a weekly audience at the Vatican dedicated to the commandment exhorting the faithful not to kill.
Francis said some people justify abortion as respecting others' rights. But he asked: "How can an act that suppresses innocent and defenceless life as it blossoms be therapeutic, civil or simply human?"
He asked if it was fair "to take out a human life" to solve a problem: "Is it fair to hire a hitman to solve a problem? It is not fair. We cannot take out a human being, even if it is small."
It was the second time within recent months that Francis has expressed the church's longstanding opposition to abortion in violent, stark terms.
In June, Francis denounced how some couples resort to pre-natal testing to see if their unborn babies have malformations and then choose to have an abortion, which he said was the "white glove" equivalent of the Nazi-era eugenics programme.
Francis has framed both abortion and euthanasia as part of what he calls today's "throwaway culture" where the sick, the poor, the elderly and the unborn are considered unworthy of protection and dignity by a society that prizes instead individual prowess and success.
Official church teaching opposing abortion is absolute, providing for no exceptions. However, Francis has acknowledged that women sometimes are driven by circumstance to abortion and he has extended the ability of ordinary priests - and not just bishops - to absolve them of the sin of abortion if they repent.
Francis's comments came during a three-week meeting of bishops from around the world on young people, where sexuality, including premarital sex, is among the topics of discussion.
Catholic teaching on abortion has been in the headlines lately, including during the divisive confirmation process of US Supreme Court Justice Brett Kavanaugh, a Jesuit-educated Catholic whose vote could overturn legalised abortion in the US.
Senators in Argentina rejected a bill in August that would have legalised the procedure in Francis's home country.
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