The true legacy of 9/11 will be "a stronger nation, and a people more united than ever before," Obama said
President Barack Obama lauded American unity as the country marked a somber but low-key anniversary of the 9/11 attacks under crisp blue skies reminiscent of the fateful day 11 years ago.
"The true legacy of 9/11 will not be one of fear or hate or division," Obama said at the Pentagon near Washington. "It will be a safer world, a stronger nation, and a people more united than ever before."
Highlighting what he said were the "crippling" blows dealt against Al-Qaeda and the killing last year of Osama bin Laden, Obama said the United States is "even stronger."
But in Egypt and Libya, angry mobs attacked US diplomatic missions on the sensitive anniversary over a film deemed an offense to Islam, killing a US official at the consulate in Libya's eastern city of Benghazi city.
Thousands protested at the embassy in Cairo, with some of the protesters scaling the walls and replacing the US flag with a black Islamic one.
As in past years, relatives of the nearly 3,000 people killed when Al-Qaeda hijackers slammed airliners into New York's World Trade Center gathered at Ground Zero to read out the names of the dead.
The pristine blue sky was identical to the one 11 years ago when millions of people watched from the streets and live on television as the planes flew straight into the upper floors of the Twin Towers, causing them to collapse.
However, emotions are distinctly cooler as America finally tries to draw a line under an event that sparked the decade-long War on Terror.
No politicians joined in the Ground Zero reading and security was less intense, in contrast to the 10th anniversary last year when Obama headed a long list of VIPs at the ceremony.
"I feel much more relaxed. After the ninth anniversary, those next days it started building up to the 10th anniversary," June Pollicino, who lost her husband on 9/11, told AFP.
"This year it's different in that regard. It's another anniversary we can celebrate in a discreet way."
In total, 2,983 names were read out at Ground Zero -- the September 11, 2001 victims and those killed in the precursor to those attacks, the 1993 car bombing of the World Trade Center.
The reading paused for silence at the exact times each of the four planes turned into fireballs -- two smashing into the Twin Towers, one into the Pentagon and another into a Pennsylvania field.
Another two moments of silence were observed at the times the towers collapsed, accounting for the vast majority of 9/11's dead.
Obama, who earlier stood for a moment's silence on the White House South Lawn, had no political events planned Tuesday and his re-election campaign planned to halt television advertising for the day.
On leaving the Pentagon, he and First Lady Michelle Obama made an unscheduled stop at Arlington National Cemetery, walking among the gleaming white crosses and laying special coins of tribute at the graves.
Vice President Joe Biden traveled to Shanksville, Pennsylvania, where United Flight 93 crashed after passengers attacked the hijackers and prevented them from hitting another presumed high-profile target, such as the US Capitol.
In a rare show of bipartisan unity, more than 275 lawmakers from the US House and Senate gathered at the Capitol steps to sing "God Bless America," much as they did in the immediate aftermath of the 9/11 attacks.
White House hopeful Mitt Romney recalled how he drove past the smoldering Pentagon on that day, and was stunned by "the smell of war" that had reached American shores. He also said he was putting aside his differences with Obama for a day to hail the men and women who protect America, and those who died.
The new memorial at Ground Zero and the near-completion of the main skyscraper at the World Trade Center, now officially the tallest building in New York, have helped to heal some of the wounds from the attacks.
The killing of Osama bin Laden in a US commando raid in Pakistan last year dealt a major blow to Al-Qaeda, and an aggressive campaign of US drone strikes on his would-be successors have further weakened the group.
The group nevertheless posted an Internet video late Tuesday accusing the United States of waging war on Islam and saying US Muslims should brace for a "holocaust," according to the SITE monitoring service.
The Taliban movement used the anniversary to scorn any notion that they are on the ropes, saying they had nothing to do with 9/11 and that the United States faces "utter defeat in Afghanistan."
Most foreign troops are scheduled to withdraw by the end of 2014, handing over responsibility for combat to Western-backed Afghan government forces.