"When we say we will never forget, we mean what we say," Obama declared at a midtown Manhattan firehouse.
"It didn't matter which administration was in, it didn't matter who was in charge," he said. "We were going to make sure that the perpetrators of that horrible act -- that they received justice."
Four days after his announcement of bin Laden's death stunned the world, Obama led a somber ceremony at Ground Zero, where a memorial and new skyscrapers are rising, nearly 10 years after the attack.
The president and a New York City firefighter laid a wreath of red, white and blue flowers at the foot of a pear tree that survived the Twin Towers' collapse.
The hushed event on the 9/11 Memorial plaza came amid a series of stops where the president met and commiserated with those who personally felt the devastation of the attacks that killed 2,752 people: police, firefighters and families of the dead.
Obama made no speeches at his first appearance at Ground Zero as president. Instead, he shook hands with a row of firefighters and police officers standing at attention in dress uniforms, thanking each one.
Then, as 9/11 family members, emergency responders and New York-area elected officials watched, Obama carried the wreath to its place on a wooden stand. He stood silently there for more than 30 seconds, head bowed, eyes closed, hands folded in front. NYPD Officer Stephanie Moses stood to his right, FDNY firefighter Joseph LaPointe, who helped the president carry the wreath, was on his left.
The cranes constructing the new One World Trade Center loomed behind him, momentarily stilled.
Fire Commissioner Salvatore Cassano, whose department lost 343 of its members on 9/11, said he choked up with tears.
"This is a day few of us will forget," he said. "It is great for the FDNY that the president was here."
Obama turned and greeted the family of Glen Wall, 38, a Lynbrook native and Cantor Fitzgerald executive who was killed in Tower One. His family, who lives in New Jersey, was chosen to stand by the Survivor Tree after his daughter, Payton, 14, wrote the president a letter about how she was coping with the loss of her father, White House officials said.
"He just told me how he was honored to be meeting with us today. I told him how honored I was to be in his presence," said Payton, who was joined by her mother, Diane, sister, Avery, 12, and friend Madison Robertson, 14, who also lost her father on 9/11.
The presidential visit came on a sunny and crisp spring afternoon during which traffic and street life came to a virtual standstill in parts of the city.
The NYPD shut down busy stretches of Manhattan streets, including traffic through Times Square, for the motorcade escorting Obama's armored Cadillac limo.
Security measures were intense. Four NYPD harbor patrol boats and the Coast Guard were on hand for the 11:07 a.m. landing of Obama's helicopter, Marine One, at the Wall Street heliport and then raced north parallel with the FDR Drive, following the motorcade.
One man, Matthew Duhaine, 30, of Chicago, was detained near Ground Zero. Police said a lieutenant at the corner of Barclay and Church streets spotted him pacing nervously with a bicycle and avoiding eye contact with officers. Officers questioned him and searched his bag, which contained a water bottle with beer, police said. No charges were filed.
Obama's first stop was a firehouse that he said symbolized the "extraordinary sacrifice" of 9/11. Engine 54, Ladder 4, Battalion 9 at Eighth Avenue and 48th Street -- dubbed the "Pride of Midtown" -- lost 15 men when the World Trade Center collapsed. Fire officials showed Obama a bronze plaque affixed to the wall, bearing the names of the dead. Obama compared their heroism to that of the Navy SEALs and CIA officers who stormed into Pakistan in a high-risk raid on bin Laden's compound.
"They were doing it in the name of your brothers that were lost," Obama said.
Obama then traveled to the NYPD's First Precinct in the Financial District, from which the first police units responded on 9/11. Obama took a group photo with about 30 officers and signed the desk sergeant's guest log book at 12:35 p.m.
"We did what we said we were going to do," Obama said of bin Laden's death. "Americans, even in the midst of tragedy, will come together, across the years, across politics, across party, across administrations, to make sure that justice is done."
But amid the triumph of a national victory, Obama reminded the officers the world remained a dangerous place.
"There are still going to be threats out there and you're still going to be called on to take courageous actions and to remain vigilant, and you're going to have an entire country behind you when you do it," Obama said.
About 60 family members of 9/11 victims met privately with the president at the 9/11 Memorial Preview Site across the street from Ground Zero after the wreath-laying ceremony. Margie Miller, 61, of Baldwin, whose husband Joel died on the 97th floor of Tower One, said she looked Obama in the eye and thanked him.
Calling bin Laden's death a moment that "transcended politics," Obama was joined Thursday by a bipartisan cast of officials.
Republican New Jersey Gov. Chris Christie, whom some in the GOP want to challenge Obama in 2012, was present for the wreath-laying, along with New York Gov. Andrew M. Cuomo, a Democrat, and Mayor Michael Bloomberg, an Independent. Obama also invited former President George W. Bush, a Republican, who declined.
And former New York City Mayor Rudy Giuliani, a Republican who drew widespread acclaim for his handling of the aftermath of the World Trade Center attacks, greeted Obama when his helicopter landed and was at the president's side much of the day. Giuliani has been a frequent critic of Obama's conduct on the war on terror.
"We may have our differences, politically, in ordinary times," Obama said of Giuliani at the First Precinct. "But when it comes to keeping this country safe, we are, first and foremost, Americans."
With Anthony M. DeStefano, Tania Lopez and Bart Jones