US President George W Bush has laid a wreath at Ground Zero, the site of New York's Twin Towers, to mark the fifth anniversary of the 11 September attack.
The president bowed after laying the wreath on the makeshift pool
Afterwards he attended a remembrance service at a nearby chapel.
The events come as Mr Bush continues to face criticism over his so-called war on terror in the run-up to November's mid-term Congressional elections.
Earlier, Secretary of State Condoleezza Rice said the US was now safer than it was before the 2001 attacks.
There were clouds over Ground Zero as Mr Bush, accompanied by his wife, Laura, walked slowly down into the hole that marks the spot where the towers once stood.
As a bagpiper played America the Beautiful, they placed floral wreaths of red, white and blue upon two dark ponds of water, set up to mark both the north and south towers, and then walked back up the ramp to street level.
Mr and Mrs Bush were joined at the site by New York Mayor Michael Bloomberg, former Mayor Rudolph Giuliani, and state Governor George Pataki.
Afterwards, the president and his wife attended a service of prayer and remembrance at St Paul's Chapel, just across the street from the site of the World Trade Center.
Mr Bush said the anniversary would be "a day of sadness for a lot of people".
He added: "I vowed that I'm never going to forget the lessons of that day. There's still an enemy out there that would like to inflict the same kind of damage again."
The simple wreath-laying ceremony marked the formal beginning of the US commemorations of the attacks on Washington and New York in which almost 3,000 people died.
At precisely 0846 (1246 GMT) on Monday a minute's silence will be observed, recalling the exact moment when the first tower was hit.
Mr Bush is expected to observe the silence with firefighters involved in rescue operations five years ago, whom he will be joining for breakfast.
At Ground Zero relatives of the 2,973 people who died will read out their names, pausing only to recall the time when the second tower was hit and when both structures collapsed.
Mr Bush will also visit the Pentagon, hit by a third plane, and the Pennsylvania crash site of the hijacked United Airlines Flight 93.
On Monday evening at 2100 (0100 GMT) he is expected to make a formal TV address to the nation from the Oval Office.
It is just the fifth time Mr Bush will make such an address to the American people - the first was on the day of the attacks.
White House spokesman Tony Snow told AFP that Mr Bush's planned address "is not a political speech".
"It is a reflection of what 11 September has meant to the president, and to the country; the realities that it has brought to all of our attention and how we can move forward together to try to win the war on terror," Mr Snow was quoted as saying.
Mr Bush's ratings soared after 11 September, but have declined with the war in Iraq and amid questions over the administration's handling of the war on terror.
Even as the president lay the wreaths at Ground Zero, crowds of protesters calling for a US withdrawal from Iraq had gathered nearby.
Correspondents say Mr Bush is engaged in a public relations offensive aimed at boosting support for the war on terror, in particular for the unpopular campaign in Iraq, in the run up to the legislative and gubernatorial elections on 7 November.
Opposition figures like Democratic National Committee Chairman Howard Dean say Mr Bush should not have diverted attention and resources from the hunt for al-Qaeda chief and 11 September mastermind Osama Bin Laden by invading Iraq.
In an interview with the Fox News network, Ms Rice defended the Bush administration's actions, saying that the US was more secure now than five years ago.
"I think it's clear that we are safer but not really yet safe," she said.
"We've done a lot... our ports are more secure, we have a much stronger intelligence-sharing operation."