He indicated on Saturday that he would uphold Pope John Paul II's unwavering stands against the two controversial issues and would work to guarantee obedience to Church doctrine.
Benedict outlined his vision of his papacy in a homily during a ceremony in which he took his place on a marble-and-mosaic throne in the ancient Roman basilica of St John in Lateran. The ceremony was the last in Benedict's formal assumption of the papacy.
His 30-minute homily, read in a calm, confident manner, reflected his experience guarding doctrinal orthodoxy and defending papal authority for two decades under John Paul.
The pope "must not proclaim his own ideas, but ever link himself and the Church to obedience to the word of God, when faced with all attempts of adaptation or of watering down, as with all opportunism", Benedict said.
"That's what Pope John Paul II did, when ... faced with erroneous interpretations of freedom, underlined in an unequivocal way, the inviolability of human beings, the
inviolability of human life from conception to natural death."
He added: "Freedom to kill is not a true freedom but a tyranny that reduces the human being into slavery."
Marshaling his arguments, Benedict said: "The pope isn't an absolute sovereign, whose thoughts and desires are law. On the contrary - the ministry of the pope is to be the guarantor of obedience to Christ and to his word."
In his former role of Vatican's watchdog on doctrine, Benedict earned a reputation as a strict interpreter of Church teaching who silenced dissident theologians.
"Freedom to kill is not a true freedom but a tyranny that reduces the human being into slavery"
Pope Benedict XVI
The installation as Bishop of Rome traditionally takes place in the ancient basilica, which the Vatican describes as the "mother and head of all the churches of the city of Rome and of the world".
Arriving at and leaving the ceremony, Benedict was cheered by thousands of Romans, who lined a boulevard as he drove by in an open-topped black limousine.
He stood and waved to the crowd as the car slowly travelled down the street.
"Dear Romans, I am now your bishop," Benedict said, concluding his homily.