New York (CNN) -- President Barack Obama marked the dedication of the long-awaited September 11 Memorial Museum Thursday with families, survivors and rescuers at the site, saying the "sacred place of healing and hope" will ensure that "generations yet unborn will never forget" the worst terrorist attack on U.S. soil.
"It is an honor for us to join in your memories, to recall and to reflect, and above all to reaffirm the true spirit of 9/11: love, compassion, sacrifice and to enshrine it forever in the heart of our nation," the President said before the gray bedrock of the fallen towers.
Both chilling and somber, the memorial will take visitors back to the day the twin towers of the World Trade Center were felled by hijacked jet planes on a clear September morning nearly 13 years ago. It also remembers those who lost their lives outside of New York the same day, when a hijacked jet flew into the Pentagon and another went down in a field in Pennsylvania.
"Here we tell their story so that generations yet unborn will never forget," Obama said. "Of coworkers who led others to safety, of passengers who stormed the cockpit, our men and women in uniform who rushed into an inferno, our first responders who charged up those stairs, a generation of service members, our 9/11 generation who have served with honor in more than a decade of war."
The sacrifices of that day and the toil to build the museum demonstrates that the United States is "a nation that stands tall and united and unafraid because no act of terror can match the strength or the character of our country," the President said.
"Nothing can ever break us," he added. "Nothing can change who we are as Americans."
The ceremony was attended by dignitaries such as New York Gov. Andrew Cuomo, former New York mayors Michael Bloomberg and Rudolph Giuliani and relatives of the more than 2,700 people who perished at the site.
Before his speech, the President and first lady Michelle Obama viewed the museum with former President Bill Clinton and former Secretary of State Hillary Clinton, along with others. Obama said the museum, which includes a mangled firetruck and a memorial wall with photos of the victims, provided a "profound and moving experience."
Speakers recounted stories of random acts of kindness and courage that marked the fateful day, of firefighters who died climbing up stairs to save lives, of 24-year-old Welles Crowther -- who emerged from the smoke wearing a red bandana and calmly led survivors to the stairs in one of the towers.
"They didn't know his name," Obama said. "They didn't know where he came from but they knew their lives had been saved by the man in the red bandana."
Crowther led survivors to safety during the chaos of the terrorist attack, before going back up the stairs to save others and losing his life. One of his red bandanas is on display in the museum.
"All those who come here will have a chance to know the sacrifice of a young man, who like so many, gave his life so others might live," Obama said.
Crowther's mother, Alison, said the memorial was a symbol of "how people helped each other that day" and she hoped it would inspire others "to do the same in ways both big and small."
She stood alongside Ling Young, one of the people her son rescued.
"It was very hard for me to come here ... but I wanted to do so, so I could say thank you to his parents," Young said.
The museum will open to the public May 21. Also memorialized inside are the victims of the 1993 bombing at the World Trade Center.
The museum and memorial plaza, which opened in 2011, were built with $700 million in donations and tax dollars following construction problems and disputes over how best to remember the thousands of lives lost that day.
The site, which has risen up from the ashes of suffering and tragedy, is expected to stand as a symbol of resilience, organizers said.
It holds some 12,500 objects, 1,995 oral histories and 580 hours of film and video.