The US is marking nine years since 9/11 amid controversy over plans for an Islamic centre near Ground Zero and a threat to burn the Koran.
The biggest event is in New York, where relatives are to read out the names of those who died when planes hit the twin towers of the World Trade Center.
Earlier, President Barack Obama urged respect for other religious faiths.
The pastor behind the threat to burn Korans in Florida meanwhile said the event had been cancelled permanently.
"We will definitely not burn the Koran, no," the Reverend Terry Jones told NBC's Today show.
"Not today, not ever," he said when pressed about whether his planned demonstration might happen at a later date.
He also said no meeting had been set up with New York Imam Feisal Abdul Rauf, who is behind the plans for the Islamic centre.
This year's anniversary is likely to be the most contentious and fraught yet, says the BBC's Laura Trevelyan.
Once the city's official commemorations are over, rallies will be held both for and against the proposed Islamic community centre and mosque near Ground Zero.
Both sides want to use the emotion of the day to highlight their causes, our correspondent says.
Four airliners were hijacked during the attacks in 2001, which claimed 2,975 lives, not including the 19 hijackers.
Two planes were crashed into the towers of New York's World Trade Center (WTC) and one into the Pentagon, while the fourth crashed into a field in Pennsylvania, after passengers fought back against hijackers.
US Vice-President Joe Biden is to attend the official ceremony in New York at which the names of all the people who died at the WTC will be read out.
Houses of worship across the city have been asked to toll their bells at 0846 (1346 GMT), the moment the first hijacked plane struck the North Tower.
President Obama will attend the ceremony at the Pentagon and his wife Michelle will attend the event in Shanksville, Pennsylvania, along with former first lady Laura Bush.
Sensitivity issue
Some relatives of the 9/11 victims says it is disrespectful to have a reminder of Islam by the place where so many died but other families support the project as an expression of America's commitment to freedom of religion.
Stephanie Parker, daughter of 9/11 victim Philip L Parker, attended a vigil in support of the Islamic centre near the site on Friday.
She said she had come to the vigil, which gathered about 2,000 people, because she was troubled by what she saw as people wrongly equating all of Islam with the hijackers.
"I think the anniversary is being overshadowed," she told the Associated Press news agency.
Sally Regenhard, who lost her firefighter son Christian Regenhard on 9/11, planned to attend both the ceremony in New York and the anti-mosque protest.
"The purpose is to speak out and express our feelings that this mosque, the location of it, is a grievous offence to the sensitivity of 9/11 families," she said.
Supporters of the planned mosque held a vigil on Friday evening
The anti-mosque rally is expected to hear taped messages from Bush-era diplomat John Bolton and conservative blogger Andrew Breitbart.
Dutch anti-Islam politician Geert Wilders, who advocates banning the Koran, plans to address the rally in person, as do a handful of Republican congressional candidates.
Saturday saw new protests in mainly Muslim countries over the Koran-burning proposal, with rallies reported in Somalia and Afghanistan.
Police escorted Mr Jones after his arrival in New York's Laguardia airport
Pastor Terry Jones had said he hoped to meet a leading imam to discuss the proposal for the Islamic centre, to be located a short distance from Ground Zero, the WTC site.
He said he had suspended the book-burning only because he had received a guarantee, from an imam in Florida, that the centre would be moved.
But the planners of the Islamic centre have said they did not speak to the Florida imam, and would not be moving their project.
Mr Abdul Rauf said on Friday that he was "prepared to consider meeting with anyone who is seriously committed to pursuing peace" but added that he had no current plans to meet Mr Jones.
Mr Jones is the pastor of the tiny and previously little-known Dove World Outreach Center in Gainesville, Florida, and author of a book entitled Islam is of the Devil.
He had planned to stage an International Burn a Koran Day on Saturday, saying the book was "evil".
But pressure was put on the pastor to cancel the burning. The FBI visited Mr Jones to urge him to reconsider his plans and he was telephoned by US Defence Secretary Robert Gates.
In his remarks on Friday, Mr Obama denied that his administration's intervention in the affair had elevated it to greater prominence.
He appealed to Americans to respect the "inalienable" right of religious freedom and said he hoped the preacher would abandon his plan to burn the Koran, as it could add to the dangers facing US soldiers serving abroad.
"This is a way of endangering our troops, our sons and daughters... you don't play games with that," he told reporters.
