The names of about 3,000 people killed in the attacks on the World Trade Centre are being read out as part of the commemoration event.
John McCain and Barack Obama, the respective Republican and Democratic contenders for the White House will make a joint appearance at Ground Zero.
George Bush, the US president, is attending a service in Arlington, Virginia to remember those killed on September 11, 2001 when a hijacked aircraft hit the Pentagon, kiling 184 people.
Another hijacked aircraft was brought down in Pennsylvania on that day, killing all on board.
Al Qaeda later claimed responsibility for the 9/11 attacks, which were carried out by 19 men on four aircraft.
The Bush administration began its so-called "War on Terror" shortly after the attacks.
|Bush's approval rating has sunk amid the so-called 'War on Terror' [AFP]
"One of the worst days in America's history saw some of the bravest acts in America's history," Bush said on Thursday at the Pentagon memorial.
"We'll always honour the heroes of 9/11 and here at this hallowed place we pledge never to forget their sacrifice."
Bush paid tribute to the US military in for campaigns in Iraq and Afghanistan.
"Since 9/11 our troops have taken the fight to the terrorist abroad so we do not have to face them here at home," he said.
"Thanks to the brave men and women and all those who work to keep us safe there has not been another attack on our soil in 2,557 days," he said.
But the US president has little support from the US public after military invasions in Iraq and Afghanistan foundered.
"I think the president's approval ratings say it all right now; they are the lowest they have ever been for a sitting president," Kristen Saloomey, Al Jazeera's correspondent at Ground Zero in New York, said.
"Also significant is the attempt made by John McCain, the Republican candidate for the presidency, to distance himself from the Bush presidency. His campaign is stressing change and a break from the Bush administration."
"Even though this week Bush announced that he is going to send 4,500 more troops to Afghanistan his critics are saying that it is too little, too late and that the focus has been lost in the so-called 'War on Terror'."
Politics 'set aside'
McCain and Obama have promised to suspend their campaigns for the anniversary.
"There will be no speeches - this is going to be a moment when politics are set aside," Linda Douglass, a spokeswoman for Obama, said.
Obama said on Wednesday that 9/11 "showed that here in America, we all have a stake in each other; I am my brother's keeper, I am my sister's keeper; and we rise and fall as one nation."
While the two candidates will stand together at Ground Zero, politicking could resume when the two candidates take part in a televised forum later on Thursday.
"The campaign's likely to get pretty nasty, but tomorrow they'll want to be above the battle [during the 9/11 anniversary]" John Mueller, a politics professor at Ohio State University, said.
"Bashing the opponent is bad politics at that moment."
The 9/11 attacks remain a deeply emotional issue in the United States, despite polls showing that US citizens are largely concerned about the state of US economy at present.
But the memory of the attacks remain raw, given that no permanent memorial at Ground Zero has yet been erected to commemorate those who were killed in the 9/11 attacks.
Sally Regenhard, whose fireman son was killed while attempting to rescue those trapped in the World Trade Centre, said she wants to see Obama and McCain focus on US security.
"I'd like to hear them say they're going to get more involved regarding... the need to protect our cities from all types of chemical and biological and radiological attacks," she told AFP.
"I want them to get involved with legislation to create a national standard for emergency response after attacks, hurricanes - whatever the emergency."
Michael Chertoff, the US chief for homeland security, said the government had undertaken a raft of initiatives to improve security.
"I don't think there's any doubt that we are safer today than we were seven years ago," he said.
But he warned that al-Qaeda remains a potent force.
"Al-Qaeda continues to focus on the aviation system as an area where they want to target," he said, arguing however, that vulnerability has been "substantially reduced."