Addressing Muslim leaders in Germany, Pope Benedict XVI delivered the strongest rebuke of terrorism of his nearly four-month-old papacy, asking Muslims to join Christians in trying to combat its spread and "turn back the wave of cruel fanaticism" behind it.

Benedict later traveled in his popemobile to the rain-soaked Marienfeld, a former coal mine near the town of Kerpen outside Cologne, for an outdoor evening service as part of the four-day Catholic youth festival.

Hundreds of thousands of high-spirited pilgrims roared their approval as Benedict arrived in his mother-of-pearl Mercedes-Benz, waving and smiling as he greeted the crowds, estimated at 800,000. Overhead, as if on cue, storm clouds that had threatened to drench the faithful began melting away, unveiling a bright blue sky.

Before giving his homily, Benedict dedicated a huge bell at the foot of the altar to his predecessor, Pope John Paul II, the man who originated World Youth Day as a Roman Catholic festival. As it tolled, a choir performed a slow hymn while the crowd sang along.

The meeting with Muslim officials in Germany was part of Benedict's outreach to non-Catholics during his visit, as he sought to achieve common positions on social issues and world peace. Germany has about 3.5 million Muslims, one of the highest figures in Western Europe.

The pope said Muslim leaders had a "great responsibility" in properly educating younger generations.

"I am certain that I echo your own thoughts when I bring up as one of our concerns the spread of terrorism," Benedict told the Muslim leadership, mainly Turks.

"Terrorist activity is continually recurring in various parts of the world, sowing death and destruction, and plunging many of our brothers and sisters into grief and despair," he said.

The pope spoke of terrorism striking in "various parts of the world" but did not mention any specific attacks, assess responsibility or speak directly about suicide bombings. It appeared significant, however, that he chose a Muslim audience for his remarks on terrorism as many recent attacks have been attributed to Islamic extremists.

Going into Saturday's meeting, the pope had been cautious about making any links between terrorism and Islam, rejecting the idea that the world faced a "clash of civilizations" and reportedly overruling an aide who wanted to brand the deadly July 7 London bombings as anti-Christian.

But in warning that the world risked exposure to "the darkness of a new barbarism," Benedict stressed that Islamic leaders must "guide Muslim believers and train them in the Islamic faith."

Benedict said that by working together, Catholics and Muslims could "turn back the wave of cruel fanaticism that endangers the lives of so many people and hinders progress toward world peace."

Ridan Cakir, president of the Turkish Islamic Union, said the participants shared the pope's position. "With this common platform, we are able together to fight terrorism," he said at a news conference afterward.

The meeting came a day after Benedict visited a Cologne synagogue to meet with Jewish leaders; he also met with Protestant and Orthodox Christian representatives.

Earlier Saturday, Benedict paid a courtesy visit to German Chancellor Gerhard Schroeder and Angela Merkel, his challenger in Sept. 18 parliamentary elections. Schroeder, who like Merkel is Protestant, as are about a third of Germans, had no public comment.

Pope Benedict XVI visits the Marienfeld, a former coal mine outside Cologne, Germany, for an evening service. Earlier, before a Muslim audience, he said the world risked exposure to "the darkness of a new barbarism."