U.S. President Barack Obama said Americans must "stay true to who we are" to pay tribute to the thousands who lost their lives in the Sept. 11 terrorist attacks, an anniversary that was marked Saturday in sombre ceremonies in New York, Pennsylvania and at the Pentagon.

Obama, Defence Secretary Robert Gates and Adm. Mike Mullen, chairman of the joint chiefs of staff, attended a service at the Pentagon, where 184 people were killed in the attacks.

Speaking to an audience of victims' families, as well as service men and women, Obama called the anniversary a day of remembrance, reflection, "and with God's grace, a day of unity and renewal."

"The perpetrators of this evil act didn't simply attack America, they attacked the very idea of America itself, all that we stand for and represent in the world," Obama said.

"And so the highest honour we can pay those we lost, indeed our greatest weapon in this ongoing war, is to do what our adversaries fear the most: to stay true to who we are as Americans, to renew our sense of common purpose, to say that we define the character of our country. And we will not let the acts of some small band of murderers, who slaughter the innocent and cower in caves, distort who we are. They doubted our will, but as Americans we persevere."

Before Obama spoke, Gates paid tribute to the soldiers who have served in the war on terror, some of whom enlisted after the attacks.

"Our troops and their families have paid a steep price these past nine years, but have also shown resilience and strength in the best traditions of the country, a country that cherishes their service and the memory of those that have fallen," Gates said. Gates also revealed that a portion of nearby Washington Blvd. is being renamed the 9-11 Heroes Memorial Highway.

In Manhattan, where 2,752 people lost their lives when two jetliners were flown into the World Trade Center buildings, thousands of New Yorkers gathered to mark the sombre anniversary.

A solo cellist played as family members read the names of the victims, as has occurred at every service since the attacks.

New York Mayor Michael Bloomberg opened the service near the site of the attacks by paying tribute to the victims and to the fortitude of those left behind.

"It was once impossible to believe that two soaring towers of people and possibilities could be so coldly silenced," Bloomberg said. "Throughout the years we have sought consolation in what we have learned. That those we lost live on in our memory, and that a measure of grace can be found in the determination to take up the light before us."

Church bells in Manhattan rang out at 8:46 a.m., the time the first plane hit the first tower.

Church bells tolled three more times, once to mark the second plane hitting the second tower, and then to mark the collapse of each building. Vice president Joe Biden spoke briefly at the New York ceremony. First lady Michelle Obama joined former first lady Laura Bush in Shanksville, Pa., where another jetliner was crashed into a field, killing everyone on board.

Prime Minister Stephen Harper issued a statement early Saturday about the anniversary.

"The tragic events of September 11 remain fresh and clear in all of our hearts and minds. The memory of that day continues to evoke horror and sorrow in all Canadians, despite the passing of nearly a decade. Today, we honour those who lost their lives during those senseless and cowardly attacks, including 24 Canadians," Harper said.

"On the anniversary of 9-11, our thoughts also turn to all those who are still grieving over the loss of their family members, friends and neighbours."

Protests cast shadow

As thousands of Americans took part in prayer services and observed moments of silence, demonstrations in the U.S. and around the world threatened to cast a pall over the events.

While the New York event is often a quiet and sombre affair, this year's events have been inflamed by debate over plans to build an Islamic centre and mosque two blocks from ground zero.

The controversy erupted in duelling demonstrations Saturday, after the morning's official ceremonies to remember Sept. 11 victims.

One group of protesters, brandishing signs with such messages as "Never forgive, never forget, no WTC mosque," cheered and jeered while another rally in support of the Islamic centre took place a few blocks away.

While family members of some of the victims planned to attend the demonstrations, many chose to stay away.

"I just wanted to be as at peace with everything that's going on as I possibly can," said Nancy Nee, whose firefighter brother was killed on 9-11 and who is opposed to the Islamic centre near Ground Zero. Nine years later, she said, her brother George Cain's death "is still very raw. ... And I just don't have it in me to be protesting and arguing, with anger in my heart and in my head."

"For the families, it's all about being there for our loved ones," said Toronto-native Stephan Gerhardt after taking part in the official ceremonies Saturday. Gerhardt, who lost his brother Ralph in the World Trade Center nine years ago, says he and many others were there to remember, not play politics.

"Today's not a day of protests, today's a day of remembering," he told CTV.

"As we get further away from September 11, 2001 more and more politics step into it. We let the other people deal with that. It's still a very private grieving that we do, even in this very public way we have."

The protests over the planned Islamic centre have corresponded with a surge in anti-Muslim sentiment in pockets of the U.S., so much so that White House officials hosted an interfaith conference earlier this week to discuss the issue.

Tensions spilled over earlier this week, when a pastor of a small Florida church earned worldwide media attention for his plan to burn copies of the Qur'an to mark the Sept. 11 anniversary.

Pastor Terry Jones backed down from his planned "Burn a Qur'an Day" after federal officials urged him to reconsider, citing in particular security concerns for U.S. service men and women overseas.

On Saturday's "Today" show on NBC, Jones confirmed that no member of his church will burn a Qur'an.

"We feel that God is telling us to stop," Jones told "Today." Asked about whether his church would ever burn the Qur'an, he said: "Not today, not ever. We're not going to go back and do it. It is totally cancelled."

Jones told "Today" his goal was "to expose that there is an element of Islam that is very dangerous and very radical," and said "we have definitely accomplished that mission."

On Thursday, Jones said he called off the event because he was assured by a Florida imam that the centre would not be built near the World Trade Center site. However, the leader of the proposed centre, Imam Feisal Abdul Rauf, said no such deal was in place. Jones said Saturday that he hopes he can meet with Rauf while he is in New York to discuss the issue.

In his address at the Pentagon, Obama alluded to the controversy.

"They may seek to spark conflict between different faiths, but as Americans we are not and never will be at war with Islam," Obama said. "It was not a religion that attacked us that September day. It was al Qaeda, a sorry band of men that perverts religion. And just as we condemn intolerance and extremism abroad, so will we stay true to our traditions here at home, as a diverse and tolerant nation. We champion the rights of every American, including the right to worship as one chooses."

International protests and remembrances

Meanwhile, thousands of Afghans took to the streets for the second-straight day Saturday to protest Jones's plan.

In Logar province, protesters set fire to shops and police checkpoints and chanted "Death to America." At least 11 people were injured in a similar protest on Friday in Badakhshan province.

In Ottawa, the 24 Canadians who died in Sept. 11 attacks were honoured in a ceremony at the Beechwood National Cemetery. A choir sang, a chaplain read a prayer and a bugler played “The Last Post,” before the assembled crowd of less than 100 people observed a moment of silence.

Brennan Basnicki, who lost his father in the attacks, was then joined by his cousin Ryan to read out the names of the Canadians killed that day.

The names of the 152 Canadian Forces personnel killed in Afghanistan were also read aloud.

"Today, as we honour the victims of 9/11 and their loved ones who still grieve, we also honour the brave men and women who put their lives on the line every day to help prevent such a terrible tragedy from happening again," Prime Minister Stephen Harper said in a statement.

With files from The Associated Press