Brother Roger Schutz, 90, a Protestant, was stabbed by an apparently deranged woman during a service at the Taize community he founded in Burgundy, eastern France, 65 years ago, local police said.
The pope called the killing "terrifying news" in his weekly audience at his Castelgandolfo summer residence south of Rome.
"This most sad news strikes me even more because just yesterday I received a letter from him," the pope said.
He said Brother Roger had written in the letter that he was sorry he would not be able to attend World Youth Day ceremonies in Cologne, Germany because of his ill health.
Archbishop of Canterbury Rowan Williams, spiritual head of the world's 77 million Anglicans, called the killing "an indescribable shock".
"Brother Roger was old but incredibly alert. There was an astonishing calm and gentleness that emanated from him. You saw it in his shining eyes, and could recognise it came straight from his heart"
Benjamin Decker, apprentice who visited Taize five months ago
He added: "Brother Roger was one of the best loved Christian leaders of our time, and hundreds of thousands will be feeling his loss very personally."
French President Jacques Chirac called the murder of Schutz - whose mixed Catholic-Protestant community is famous for its
meditative modern church music - shocking and tragic news.
"All his life, this man of faith had a message of peace and dialogue," he said.
"For us, and especially for the young, he will remain one of the most remarkable servants of the values of respect and tolerance which are the foundation of human communities," Chirac said.
Police said a woman stood up during a religious ceremony attended by some 2500 people and stabbed Schutz three times in the throat. He died immediately.
The 36-year-old Romanian woman was overpowered by worshippers and detained.
Jean-Louis Coste, the public prosecutor in the town of Macon, close to Taize, said the suspect "appears disturbed".
But the woman, named only as Luminita, spoke fairly coherently and was fit for interrogation, he said.
Pope Benedict (red) gave Brother
Roger, a Protestant, communion
She was due to undergo further psychiatric tests later on Wednesday.
"She said that she didn't intend to kill him but that she wanted to attract his attention. She admits the facts [of the killing)]" the prosecutor said.
Coste said the woman, who arrived at the Taize community on Sunday by car, had bought the knife with which she stabbed Brother Roger on Monday at the town of Cluny, a few kilometres away, suggesting premeditation, he said.
A murder investigation was to be launched on Thursday.
Brother Roger, a Protestant pastor from Switzerland, enjoyed great respect among religious leaders.
He founded the monastic community in Taize in 1940 after
leaving Switzerland at 25 and buying a small house in the
village, where he welcomed refugees fleeing the second world war.
Today the Taize community he ran is made up of over a
hundred brothers, including Catholics and Protestants, from more than 25 nations.
The community worked to reconcile divided Christians and separated peoples.
Hundreds of Catholics paid their
respects at Cologne's St Agnes
At his inaugural Mass last April, Pope Benedict gave Brother Roger communion, making a surprising exception to the rule he has long defended that Protestants cannot take the Catholic eucharist or holy communion.
Aides said this showed how close the two were.
Thousands of young Catholic and Protestant people flock to Taize each year to meet, pray and sing together.
Hundreds of young Catholics gathered in Cologne, Germany for World Youth Day crowded into St Agnes Church, which was hosting the Taize community members.
"It was such a terrible shock, he was such a powerful presence. Many of us broke down in the church last night," said Benjamin Decker, a 19-year-old apprentice from Cologne who visited Taize five months ago.
"Brother Roger was old but incredibly alert. There was an
astonishing calm and gentleness that emanated from him. You saw it in his shining eyes, and could recognise it came straight from his heart," he said.