Tensions flared Saturday over plans to build a mosque near ground zero as demonstrations began shortly after family members of Sept. 11 victims recited loved one's names through tears at a sombre ceremony marking the ninth anniversary of the terrorist attacks on the U.S.

After the ceremony, around 1,000 activists rallied about five blocks from the site of the 2001 attacks to support the proposed Islamic community centre. Opponents gathered blocks away, with the two groups expected to converge near the mosque site.

Speaking at "hallowed ground" at the Pentagon, President Barack Obama alluded to the controversy over the mosque -- and a Florida pastor's threat, later rescinded, to burn copies of the Qur'an, the Muslim holy book. Obama made it clear that the U.S. is not at war with Islam and called the al-Qaida attackers "a sorry band of men" who perverted religion.

"They may seek to exploit our freedoms, but we will not sacrifice the liberties we cherish or hunker down behind walls of suspicion and mistrust. They may wish to drive us apart, but we will not give in to their hatred and prejudice," Obama said, despite the terrorists' efforts to spark conflicts among faiths. "As Americans we are not -- and never will be -- at war with Islam."

Family members gathering at observances in New York and Pennsylvania brought flowers, pictures of loved ones and American flags, but no signs of opposition or support for the mosque. As they read victims' names, they urged a restrained tone.

"Let today never, ever be a national holiday. Let it not be a celebration," said Karen Carroll, who lost her brother, firefighter Thomas Kuveikis. "It's a day to be sombre; it's a day to reflect on all those thousands of people that died for us in the United States."

Toting signs saying, "The attack on Islam is racism" and "Tea Party Bigots funded by corporate $," mosque supporters gathered near City Hall about an hour after the New York anniversary ceremony ended and planned to march closer to ground zero.

Elizabeth Meehan, a 51-year-old Christian from Saratoga, New York, rode in by bus from her home 180 miles (290 kilometres) away.

"I'm really fearful of all of the hate that's going on in our country. People in one brand of Christianity are coming out against other faiths, and I find that so sad," she said. "Muslims are fellow Americans; they should have the right to worship in America just like anyone else."

Just blocks away, demonstrators chanting "USA, USA" gathered to protest plans to build the Islamic centre near ground zero. Some carried signs, including messages like "Never forgive, never forget, no WTC mosque."

At the anniversary ceremony, stifling sobs in front of microphones, some Sept. 11 family members who read names sought to emphasize sentiments on all sides of the mosque argument.

Some -- like Elizabeth Mathers, whose father, Charles Mathers, worked at Marsh & McLennan at the trade centre -- stressed that ground zero is hallowed.

"New York, please be mindful this is a sacred site and should be respected as such," she said.

Many sought to embrace unity and a spirit of reaching out, which is what the developers of the Islamic centre have said is their goal.

"May we share your courage as we build bridges with other people to prevent this from happening again and to preserve human dignity for all," said Robert Ferris, saluting the dozens of construction workers rebuilding at ground zero who joined families in reading names.

Ferris lost his father, who worked at Aon Corp.

Bagpipes and drums played to open the ceremony, followed by brief comments by Mayor Michael Bloomberg.

"Once again we meet to commemorate the day we have come to call 9/11. We have returned to this sacred site to join our hearts together, the names of those we loved and lost," Bloomberg said. "No other public tragedy has cut our city so deeply. No other place is as filled with our compassion, our love and our solidarity."

Vice-President Joe Biden also spoke at the New York ceremony, where 2,752 people were killed when two jetliners flew into the trade centre.

Moments of silence were held at 8:46 a.m., 9:03 a.m., 9:59 a.m. and 10:28 a.m. to mark the times the hijacked jetliners hit the north and south towers of the World Trade Center, as well as the times they collapsed.

Hundreds of family members later placed roses in a reflecting pool at ground zero in front of a memorial, leaving scrawled remembrances on paper around it. Visible behind the podium of mourners were the beginnings of two skyscrapers rising at the site along with a transit hub.

Laura Bush, first lady at the time of the attacks, joined current first lady Michelle Obama at a service in Shanksville, Pennsylvania, to honour the 40 victims of Flight 93, who fought back against the hijackers who crashed the plane into a field there, while the president attended the service at the Pentagon, where 184 people died.

"May the memory of those who gave their lives here continue to be an inspiration to you and an inspiration to all of America," Michelle Obama said, thanking Bush for helping the country through the aftermath of Sept. 11.

The mosque debate pits advocates of religious freedom against critics who say putting an Islamic centre so close to ground zero disrespects the dead. While the rallies in New York embroiled victims' family members in a feud over whether to play politics, a threat to burn copies of the Qur'an was apparently called off.

Terry Jones, the Florida pastor who made the threat, flew to New York on Friday night and appeared Saturday on NBC's "Today" show. He said his church would not burn the Qur'an, a plan that inflamed much of the Muslim world and drew a stern rebuke from Obama.

"We feel that God is telling us to stop," he told NBC. Pressed on whether his church would ever burn the Islamic holy book, he said: "Not today, not ever. We're not going to go back and do it. It is totally cancelled."

Lending credence to Jones' comments, a "Burn a Qur'an Day" banner outside his Florida church was taken down.

Still, protests continued Saturday in Afghanistan. Police fired warning shots to prevent protesters from storming the governor's residence in Puli Alam in Logar province, officials said. Villagers set fire to tires and briefly blocked a highway to Pakistan, a provincial spokesman said.

Jones said that he flew to New York in the hopes of meeting with leaders of the Islamic centre but that no such meeting was scheduled. Imam Feisal Abdul Rauf, leader of the planned mosque, had also said Friday he had no meeting planned with Jones.

Activists in New York insisted their intentions were peaceful. The demonstrations were expected to meet near the mosque building, a former clothing store two blocks north of the trade centre site.

John Bolton, who was U.S. ambassador to the United Nations under President George W. Bush, was expected to send a videotaped message of support to the anti-mosque rally, as was conservative blogger Andrew Breitbart. Anti-Islam Dutch politician Geert Wilders, who advocates banning the Qur'an and taxing Muslim women who wear head scarves, planned to address the crowd in person, along with a handful of Republican congressional candidates who have made opposition to the mosque a centerpiece of their campaigns.

Muslim prayer services are normally held at the site, but it was padlocked Friday and closed Saturday, the official end of the holy month of Ramadan. Police planned 24-hour patrols until next week. Worshippers on Friday were redirected to a different prayer room 10 blocks away.