Cardinals announced his name after a conclave that lasted a little more than 24 hours.
The German Cardinal, who has taken the name Benedict XVI, is known to be an uncompromising conservative.
Ratzinger head the Congregation for the Doctrine of the Faith, the church's chief think tank that has dominated discussions on sexual morality and birth control and prevented liberals from gaining ground.
Ratzinger, 78, is one of the few members of John Paul II's inner circle.
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