Pope Benedict XVI has warned of rising anti-Semitism as he visited a synagogue in Cologne, in his native Germany.
The new Pope is hoping to reach out to other faiths
Condemning the "unimaginable crime" of the Holocaust, he joined in prayers before a memorial to the six million Jews killed by Nazi Germany.
The visit was only the second time a head of the Catholic Church has visited a Jewish place of worship.
The Pope is on the second day of a trip originally scheduled for Pope John Paul II, who died in April.
Addressing Jewish leaders at the synagogue, Pope Benedict said: "Today, sadly, we are witnessing the rise of new signs of anti-Semitism and various forms of a general hostility toward foreigners.
"How can we fail to see in this a reason for concern and vigilance?"
The synagogue - destroyed by the Nazis in 1939 and reconstructed 20 years later - contains a memorial to the Jews who died in the Holocaust, of whom 11,000 lived in the city.
The Pope said progress had been made in improving relations between Catholics and Jews, but that "much more remains to be done".
"We must come to know one another much more and much better," he said.
Pope Benedict's visit follows Pope John Paul II's decision to enter the Rome synagogue in 1986.
Rabbi Alan Plancey of the UK's Chief Rabbinate welcomed the visit as "an important symbolic moment" in relations between Catholics and Jews.
"It is imperative that we continue to talk to each other, and learn from the past to improve our shared future," he said.
Later on Friday, the new Pope will meet representatives of the German Protestant Churches. During his four-day stay in Cologne, he also plans to meet Muslims.
The Pope plans to make clear that he regards the creation of better relations with all religions as an essential step on the road towards seeking world peace, says the BBC's Rome correspondent David Willey.
About 400,000 Christians are in Cologne for a Catholic World Youth Festival. Their numbers are expected to double when the Pope preaches at an outdoor mass on Sunday.
The World Youth Day festival, invented by the late Pope, is held in a different part of the world every three years.
Arriving on Thursday, the Pope said he wanted to reinvigorate Christianity in an increasingly secular Europe.
The Pope has frequently bemoaned the waning role of the Church in Europe and says he hopes his trip will help kick-start "a wave of new faith among young people".
Vatican observers will be watching to see what sort of relationship he is able to establish with young Catholics, our correspondent says.
Many of them have been openly critical of the prohibitions he issued during the 20 years when he headed the Roman Catholic Church's disciplinary body.