President Barack Obama Thursday morning marked the opening of the 9/11 memorial and museum, where one of the darkest moments in the nation's history is illustrated in overwhelming detail and brutal honesty.

"Here at this memorial, this museum, we come together, we stand in the footprints of two mighty towers, graced by the rush of eternal waters, we look into the faces of nearly 3,000 innocent souls, men and women and children of every race, every creed, from every corner of the world, we can touch their names and hear their voices," Obama said, 4,629 days after the attacks.

Sept. 11 survivors, victims' loved ones and rescuers attended the dedication of the long-awaited museum.

"It is a witness to tragedy. It is an affirmation of human life," said former Mayor Michael Bloomberg, chairman of the National September 11 Memorial & Museum, which operates the museum.

Obama and first lady Michelle Obama toured the 9/11 museum Thursday morning, and it will open to the public May 21.

"We can touch their names and hear their voices, glimpse the small items that speak to the beauty of their lives -- a wedding ring, a dusty helmet, a shining badge -- here, we tell their story so that generations yet unborn will never forget," the president said.

Before the keynote address, the first couple saw the exhibits up close with Hillary and Bill Clinton, Bloomberg, and Bloomberg's companion, Diana Taylor.

Their route to the deep underground space, which was just recently completed, smelled of fresh paint.

They saw three pieces of wreckage recovered from the attack site, a destroyed fire engine, and artifacts such as the North Tower's antenna and an elevator motor.

During the ceremony, Mayor Bill de Blasio described the "ordinary everyday objects" on display at the museum -- a wallet, a ring, an ID card, a telephone, a pair of shoes -- that "are unlikely but powerful keepsakes which help us understand the events of that day in human terms."

"Each piece carries with it another story, one that might have been our own -- for don't we all own a pair of shoes we wear to work that could have been the ones we wore that day?" de Blasio said.

Rudy Giuliani, mayor at the time of the attacks, recalled the senselessness of that day.

"We will never understand why one person escaped, and another didn't -- how random it all seems, how powerless it makes us all feel," Giuliani said.

The hourlong ceremony featured survivors who managed to escape with their lives -- and stories of heroes who paid with theirs.

Obama recounted how moments after the attacks a stranger in a red handkerchief -- later identified as Welles Crowther, 24, an equities trader -- rounded up survivors, shouted for fire extinguishers, tended to the wounded, led the survivors down the stairs, then climbed back up to rescue more people. Crowther lost his life that day.

On Thursday, his mother, Allison, appeared on the podium hand in hand with one of those her son saved, Ling Young, a tax auditor.

The stories of Welles Crowther, Ling Young and Allison Crowther were just three of hundreds told at the museum.

At a news conference Wednesday, museum officials debuted the long-awaited museum that has been shrouded in controversy practically from its inception.

"It was never easy, but essential," Bloomberg said. "This museum was built to be a landmark that will withstand the test of time."

He said opening the museum "is an important day for New York, the nation and the civilized world."

Joe Daniels, the museum's president, said it is a symbol "of heroism that people will see around the world what this city, this nation did to help those in need. . . . These ties that bind us strengthen us under the most unimaginable odds, something that the terrorists did not know."

And Alice Greenwald, the museum's director, said when visitors see the mangled fire trucks, ambulances and personal belongings recovered in the debris and rubble, "they will wonder what happened to those people; to the firefighters who were in that engine; whether they survived and maybe what they were thinking on that day. . . . The museum is about people."

Visitors begin the 2½-hour tour by descending seven stories below ground level. They hear the anxious telephone calls made in the early minutes of the attacks, from people in disbelief who witnessed the planes separately crash into the Twin Towers, to a person who sees it on television from a coffee shop in Knoxville, Tennessee.

"I couldn't believe what I was watching," a voice that echoes through a dark corridor explains. The words are projected onto screens with photographs of people's horrified faces staring into the sky.

Visitors continue their descent to view the famous slurry wall that continues to hold back the waters of the Hudson River in the west chamber.

Adjacent, shiny surfaces emerge -- the two tower volumes. The aluminum-surfaced walls together are "a memory of the towers, which occupies the same space where the north and south towers once stood," said Carl Krebs of Davis Brody Bond, the museum's lead architect. Walking farther along the wide, wooden ramp, visitors can walk up to rusted, steel remnants of the towers. On the corridor walls are photos of the burning Twin Towers after their attacks by hijacked jetliners, and projected images of missing people, fliers that covered the city's public places posted by loved ones who searched for survivors.

Allen Miller of Port Jefferson, a construction manager at the museum, said crews have come up with a saying that reflects the museum -- "Stronger Than Before."

"People will come out of here feeling a sense of pride -- feeling stronger than before," he said.

Behind a memorial wall with a quote from Virgil -- "No day shall erase you from the memory of time" -- are photographs of those who died and the controversial placement of unidentified human remains recovered from Ground Zero.

An exhibition room includes thousands of artifacts displayed, each telling stories of the recovery, cleanup and aftermath of Sept. 11, including film footage of construction grapplers sifting through debris to encased dust-covered shoes of those who were caught in the ash and debris that swept through lower Manhattan. Several controversies have marked the museum's creation.

Top among them is the placement of human remains inside the museum. Some families who lost loved ones in the attacks say it is inappropriate to keep the remains of their loved ones in a museum, though other families favor the plan. A bronze sign reads: "Reposed behind this wall are the remains of many who have perished at the World Trade Center site on September 11, 2001."

Other controversies range from the $24 admission fee that will finance the museum's annual $60 million budget, to a movie that will be shown that Muslim groups say uses "anti-Islamic terminology" rather than criticizing the terrorists, who were Muslim.

"The Rise of Al Qaeda" is narrated by NBC anchor Brian Williams. Only time will tell whether museum architects and decision-makers made the right choices when creating the museum, said Daniels, noting the differences of opinions that have surfaced. "But I know once this city and the country sees this museum, it will be something that will make people feel proud."

With Maria Alvarez