US President George W. Bush, who launched the global war on terrorism and the invasion of Iraq in response to the attacks, kicked off two emotional days of global commemorations by laying wreaths at Ground Zero.
Accompanied by his wife, Laura, president Bush laid flowers on the site where each of the World Trade Centre towers once stood as bag pipes played in the background.
A few dozen protesters greeted president Bush, who has seen his approval rating dip sharply as the unpopular war in Iraq continues and a divided US public worries whether it is safer five years after the devastation wrought by the Al-Qaeda network of Osama bin Laden, the terrorist mastermind who remains at large.
President Bush called for flags to fly at half mast and for people across the country to observe a moment of silence at 8:46 am (1246 GMT), the exact time that the first of two planes ploughed into the World Trade Centre.
The president will also lay a wreath in the Pennsylvania field where a third jet crashed after passengers fought back against their hijackers, killing 40 people.
He will then fly to the Pentagon for commemorations there.
Roll call
In what has become an annual ritual, husbands, wives and partners of the 2,749 people who perished in the World Trade Centre will read a roll call of the dead, pausing only to mark the moments the planes struck and the towers collapsed.
As evening falls, two giant beams of light symbolising the collapsed towers will illuminate the Manhattan sky. A candle-lit vigil is planned at Ground Zero.
For only the fifth time in his presidency, president Bush will deliver a televised address to the nation from the White House in the evening, in what has been billed as "a non-political speech about what September 11 has meant to the nation."
In the run-up to the anniversary, president Bush made several speeches justifying decisions made in the name of the so-called "war on terror."
As in previous years, Arab television broadcast new al-Qaeda video footage just days ahead of the anniversary, this time purportedly showing bin Laden and two of the 19 hijackers preparing for the attacks.
Iraq and al-Qaeda
On the eve of the anniversary, US administration officials admitted that ousted Iraqi leader Saddam Hussein was not behind the al-Qaeda attacks on US targets but defended the decision to invade, insisting Saddam was linked to the network.
"We've never been able to confirm a connection between Iraq and 9/11," Vice President Dick Cheney said on NBC, but said that a connection with al-Qaeda was "different issue."
"There are two totally different propositions here. People have consistently tried to confuse them," he said, calling Saddam a state sponsor of terror and noting that al-Qaeda leader Abu Musab al-Zarqawi was in Iraq before the US invasion.
Despite the billions of dollars spent on the military campaigns, bin Laden, believed to be hiding in the rough, mountainous region straddling the Afghanistan-Pakistan border, remains at large, and the Washington Post has reported that his trail had grown "stone cold."
The newspaper, citing unnamed US and Pakistani officials, said no tips, human or electronic, had led them anywhere near the al-Qaeda leader.
Location 'unknown'
Secretary of State Condoleezza Rice acknowleged that the United States did not know bin Laden's precise location, noting that he is in "apparently, very remote areas."
"He doesn't communicate, apparently, very much," she said in an interview on Fox News. "And it is not easy to track someone who is determined to hide in very remote areas."
But, she said, there are "fewer and fewer places for him to hide."
However, Ms Rice said the US had "badly hurt" the al-Qaeda movement and the US was clearly a safer place than the day five years ago.
"I think it's clear that we are safer, but not really yet safe. We're more secure, our ports are more secure, our airports are more secure,” she said.
"We have a much stronger intelligence sharing operation, not just within the country where we've broken down walls between law enforcement and intelligence agencies to get all of the information to break up terrorist plots, but also across the world."
"We've clearly hurt badly the al-Qaeda organisation that planned and plotted and executed September 11, capturing many of their major field generals.”
She specifically referred to Khalid Sheikh Mohammed, the self-proclaimed architect of the September 11 attacks, and suspected bin Laden aide Abu Zubaydah who were moved last week from secret CIA custody to the US military prison at Guantanamo Bay, Cuba.
Pledge from Brussels
Meanwhile the European Commission's two top officials have pledged that Europe will further increase its cooperation with the United States on counter-terrorism.
"Five years on we are still fighting together the scourge of terrorism. But we have come a long way and are better prepared to respond than we were five years ago," Commission Vice-President Franco Frattini said in a letter to Michael Chertoff, US secretary for homeland security.
"Europe has suffered major attacks but has been able to thwart several attempts to cause more loss of innocent lives by terrorists. The most important lesson learnt has been that the key to success in our fight against terrorism is co-operation with our allies and partners," Mr Frattini said.
European Commission President Jose Manuel Barroso, meanwhile, said that while the EU's counter-terrorism strategy will continue to deliver results, more remained to be done such as fighting radicalisation and recruitment.
Sarkozy salutes firefighters
French Interior Minister Nicolas Sarkozy hailed New York firefighters as the heroes of September 11 in a New York ceremony marking the fifth anniversary of the attacks.
"On September 11, 2001, you, the firefighters of New York, you showed the entire world the meaning of heroism," Mr Sarkozy said at the home of Engine 54-Ladder 4-Battalion 9, which lost the most members in the World Trade Centre disaster.
"Thanks to you, thousands of innocent people are alive today," Mr Sarkozy said. "And to you, I want to say that like all the children of killed firefighters, 'Your fathers were heroes.'," he told 10-year-old Aiden Fontana, whose father died that day.
Mr Sarkozy, a likely candidate for president in France in 2007, noted that there were five French citizens among the almost 3,000 killed in the September 11, 2001 attacks.