Thousands will gather at ceremonies at the site of the destroyed World Trade Center towers, at the Pentagon, and in a field in Pennslyvania where the fourth hijacked plane crashed.
President Barack Obama and former president George Bush, who was leading the country at the time, will lead the remembrances for the 2,977 people who died.
New York's 650lb Bell of Hope, which hangs near Ground Zero in St Paul's Chapel, the rest centre for 9/11 rescue workers, will sound at 8.46am New York time, the moment of the first impact.
The two presidents will join victims' families to hear the reading of the names of those who died as bells toll across the city and President Obama will also visit the other two sites.
The remembrance takes place in the shadow of the "credible" threat of a terror attack on the mainland US as other ceremonies begin to take place around the globe.
In the UK, dignitaries and families of those who died will attend the September 11 memorial garden next to the American Embassy in Grosvenor Square, London.
A service will be held at St Paul's Cathedral this morning and at Westminster Abbey this evening.
Some families of British 9/11 victims attended an early morning service of "remembrance and reconciliation" in the Grosvenor Chapel in central London, which is known as the "American Church" after it became a place of worship for US soldiers in the Second World War.
Courtney Cowart, a survivor who was nearly buried alive when the north tower of the World Trade Center collapsed on top of her, described her fears when she returned to the site for a church service five days later.
"I was terrified. We were dwarfed by immense wreckage looming around us. It was a landscape drained of all colour," she told the congregation.
Barbara Stephenson, the Deputy US ambassador to Britain, lit a candle during the service to symbolise the banishment of darkness and everlasting life.
In Kabul, the US ambassador, Ryan Crocker, said America must remain in Afghanistan “for the long haul”.
Ambassador Crocker, who flew into New York after hearing of the attacks and saw both towers fall, said: “For me the last 10 years have always been about 9/11. I keep in my office in a small frame the boarding pass I have from that flight.
“I will never forget what happened on that day and I will never give up on my commitment to do everything I can to ensure 9/11 never happens again.”
The ceremonies ranged from the formal and grand to the intimate memorial of a single grieving mother.
In Malaysia, Pathmawathy Navaratnam wished her dead son: "Good morning." Vijayashanker Paramsothy, a 23-year-old financial analyst, was killed in the attacks on New York.
His mother said: "He is my sunshine. He has lived life to the fullest, but I can't accept that he is not here anymore. I am still living, but I am dead inside."
In the northeastern Indian state of Manipur, 100 family members and close friends of Jupiter Yambem gathered to remember the manager of the Roof of the World restaurant in the World Trade Center.
Players from the American Eagles rugby team were among the first to mark the anniversary at a memorial service in the town of New Plymouth in New Zealand.
The players, who are participating in the Rugby World Cup tournament, listened to a speech by David Huebner, the US ambassador, whose brother Rick survived the attacks.
In Japan, families gathered in Tokyo to pay their respects to the 23 Fuji Bank employees who never made it out of their World Trade Center office. A dozen of the workers who died were Japanese.
Security measures in the two US cities were tightened following uncorroborated but "credible" intelligence suggesting that the leaders of al Qaida could have dispatched operatives into the US to set off a lorry bomb.
Metal barriers were mounted along roads surrounding the site of the World Trade Center in lower Manhattan, while police have been ordered to patrol bridges looking for suspect vehicles.
In his weekly radio address, President Barack Obama yesterday called for national unity as he attempted to reassure the nation ahead of the anniversary of the worst attacks the country's mainland has ever experienced.
President Obama said: "We are doing everything in our power to protect our people. And no matter what comes our way, as a resilient nation, we will carry on.
"The terrorists who attacked us that September morning are no match for the character of our people, the resilience of our nation or the endurance of our values."
Details of the latest alleged terrorist plot are sketchy but US officials said a CIA informant had told them of a plan involving three men of Arab descent, two of whom could be American citizens and already in the country.
The third suspect is thought to have travelled to Europe but not entered the US.
They had been instructed to set off a car or lorry bomb in Washington or New York City.
American authorities had been warned of the plot, supposedly ordered by Ayman al Zawahiri, the man who replaced Osama bin Laden as head of the al Qaida terrorist network.
It is understood that intelligence agencies in the US have been unable to corroborate the informant's claims.
The ceremonies in New York began on Saturday, as thousands of local residents, for whom 9/11 was an attack on their home as well as on a symbol of American might, linked hands with visitors from across the country and around the world as the nation began an emotionally-exhausting weekend of ceremonies and services marking the 10th anniversary of the atrocities.
Rising in front of them as they reached out to friends, neighbours and strangers, was the new World Trade Center tower, soaring into the void left that day and destined to be America's tallest building.
Ten years ago on September 11, Laura Morrison fled her home a few blocks from the WTC with her two-year-old daughter in a push-chair along the same footpath where they gathered on Saturday, with the Statue of Liberty symbol of freedom standing tall and proud in the harbour behind them.
She was pregnant at the time with her second child. Now they returned as a family - Mrs Morrison, her husband Scott, and their daughters Zoe, 12 and Jamie, nine. "We live here, we work here, we are raising our family here and we are not going to be driven away," she said.
Their defiance was only deepened, they said, by threat of another al-Qaeda attack
President Bush, the country's leader at the time of the attacks but who has maintained a low profile since leaving office, took centre stage in two ceremonies yesterday.
At the Pentagon, he led a brief and sombre silent tribute to the 184 people killed when a hijacked plane ploughed into the defence headquarters just outside Washington. Joined by his wife, Laura, he laid a wreath of white flowers by the memorial stone embedded in the wall where the plane struck.
Mr Bush later joined Bill Clinton, the former president, and Vice-President Joe Biden at a ceremony dedicating the marble "wall of names" near Shanksville, Pennsylvania. It was there that an onboard rebellion by passengers and flight crew foiled plans by the hijackers of United 93 to fly into the US Capitol or the White House, forcing them down instead into a remote field.
The Foreign Office urged British citizens in America to be vigilant in the light of the latest threat.
The 9/11 attacks claimed the lives of 2,977 innocent people, making them the deadliest terrorist atrocity America has ever experienced.
Some 2,753 died in New York, 184 in Washington and 40 in Pennsylvania. Among the dead were 343 firefighters, 37 police officers, two FBI officers and one US secret service agent.
