On April 11, 1945, at 3:15 p.m., the eldest inmate of the Nazi concentration camp Buchenwald stepped in front of a loudspeaker, an instrument that until then had only broadcast orders of the SS, and spoke four words: "Comrades, we are free."

That same day, U.S. troops had liberated this camp near Weimar, in eastern Germany. For an estimated 56,000 inmates, help came too late -- they had already died at the hands of the Nazis. But the clock at Buchenwald is still there today. Its hands are frozen in the position of 3:15 p.m.

On Friday, Barack Obama became the first U.S. president to visit Buchenwald. The visit came a day after he had reached out to the Muslim world in a speech in Cairo, and just hours after he and German Chancellor Angela Merkel renewed calls for urgent action to bring peace to the Middle East.

Flanked by Merkel and former Buchenwald inmates Bertrand Herz and Eli Wiesel, a Nobel laureate, Obama entered the camp through its large gate building, which the Nazis had marked with the cynical line "Jedem das Seine" -- "To each his own."

They paused to lay white roses at a memorial tablet noting the nationalities of the 250,000 people that had been detained here in total -- Jews, but also political dissidents, homosexuals, Sinti and Roma -- and then walked across the vast space where the wooden barracks that housed them once stood. By the end of World War II, Buchenwald was the largest concentration camp on German soil.

The group continued on to the so-called Kleines Lager (small camp), where thousands of people died in the final weeks before the liberation. Here, inmates too weak to work were rounded up in stables.

"They were made for 50 horses, but they put us in, hundreds of us," Arek Hersh, a survivor, told ARD television. "And so you died."

Wiesel's father died here in Buchenwald, and the Nobel laureate later said his grave was "somewhere in the sky, which has become … the largest cemetery of the Jewish people."

Obama appeared shaken and paused many times to ask questions of the survivors. The group finished the tour of Buchenwald at its dark heart, the crematorium, where the bodies were burned.

The president's great-uncle, Charles Payne, was among the U.S. troops who liberated Ohrdruf, a nearby satellite camp to Buchenwald. Obama has previously said that Payne has had difficulties dealing with what he saw there.

"More than half a century later, our grief and our outrage have not diminished," Obama said after the tour. "I will not forget what I have seen today."

Obama added that Buchenwald was the "ultimate rebuke" to those people still denying the Holocaust, "a denial of fact and truth that is baseless and ignorant and hateful."

Merkel spoke of "the obligations placed on Germany as a result of our past" and vowed that "we Germans are united by the strong will that such a thing can never happen again."

But Wiesel reminded both leaders that those had been empty promises.

"Had the world learned, there would have been no Cambodia and no Rwanda and no Darfur and no Bosnia," Wiesel said. "Will the world ever learn?"

In a joint news conference with Merkel earlier Friday in Dresden, Obama was asked whether he had done enough to stop genocide in Sudan's Darfur region.

Obama said he had appointed one of his key national security advisers, Gen. Scott Gration, as a special envoy to the conflict there. Sudanese President Omar al-Bashir has expelled humanitarian workers from the country where aid groups have warned genocide continues.

Gration is tasked with trying to "not only solve the immediate humanitarian crisis that exists," but also to "reactivate the possibilities of a peace settlement between Khartoum and some of the rebels in Darfur that would allow the internally displaced people from Darfur to start returning to their homes."

Earlier Friday Obama toured Dresden's historic city center and attended a ceremony in the Church of our Lady, a baroque jewel that was destroyed by Allied firebombing in 1945 but was later rebuilt thanks in part to U.S. donors.

"All of humanity hopes for peace in a place like this where everyone can see that reconciliation between former enemies is possible," Bishop Jochen Bohl said at the ceremony.

From Buchenwald, Obama flew to Ramstein Air Base and Landstuhl Medical Center to meet with U.S. troops wounded in Afghanistan and Iraq. He will continue on to France to attend ceremonies marking the 65th anniversary of the D-Day landings.