He used a homily at a Mass before the conclave to issue a stern warning that godless modern trends must be rejected. The address was widely seen as promoting his candidacy.
He was expected to take a tough line against reformist trends in Europe and North America. In a Good Friday Mass this year he said: "How much filth there is in the Church, even among those who, in the priesthood, should belong entirely to Him."
Ratzinger's stern leadership of the Congregation of the Doctrine of the Faith, the modern successor to the Inquisition, delighted conservative Catholics but upset moderates and other Christians whose churches he described as deficient.
Before St. Peter's bells confirmed Benedict's election, there were 10 minutes of confusion over the color of the smoke, which initially seemed grey.
But even before the bells pealed, thousands of faithful in the square cheered and applauded, yelling "A pope, a pope!"
It was only the third time in a century that a pope had been chosen on the second day of a conclave. The new Pope had to win a two-thirds majority of the 115 red-robed cardinals.
NEW POPE TOUGH DISCIPLINARIAN
As John Paul's doctrinal overseer, Ratzinger disciplined Latin American "liberation theology" theologians, denounced homosexuality and gay marriage and pressured Asian priests who saw non-Christian religions as part of God's plan for humanity.
In a document in 2000, he branded other Christian churches as deficient -- shocking Anglicans, Lutherans and other Protestants in ecumenical dialogue with Rome for years.
Ratzinger was the oldest cardinal to be named pope since Clement XII, who was also 78 when he became pope in 1730. He is the first German pope since Victor II (1055-1057).
Before the conclave door shut on Monday, Ratzinger made a final appeal to his fellow electors to protect traditional teachings and to shun modern trends.
He made no mention of the challenges that other cardinals and ordinary Catholics say should top the agenda such as poverty, Islam, science, sexual morality and Church reform.
Born in Bavaria on April 16, 1927, Ratzinger was a leading theology professor and then archbishop of Munich before taking over the Congregation for the Doctrine of the Faith in 1981.
(Additional reporting by Clara Ferreira-Marques, Phil Stewart and Jane Barrett in Vatican City)
