Survivors mourned at New York's Ground Zero on Thursday as John McCain and Barack Obama suspended their rancorous White House campaign to commemorate the seventh anniversary of the 9/11 attacks.

A moment of silence began ceremonies at the side of the giant pit where the World Trade Centre towers once stood, followed by the reading of almost 3,000 victims' names.

New York Mayor Michael Bloomberg said September 11, 2001, when al-Qaida-hijacked airliners demolished the Twin Towers and also crashed into the Pentagon and a field in Pennsylvania, was the day the United States "broke."

The anniversary, he said, was about "New Yorkers, Americans and global citizens remembering the innocent people from 95 nations and territories that lost their lives that day."

McCain and Obama, due to visit Ground Zero, after the official ceremonies, declared a truce and suspended advertising.

"There will be no speeches," Democrat Obama's spokeswoman Linda Douglass said. "This is going to be a moment when politics are set aside."

Victims' relatives, some choking on tears, read out names of the dead. A string quartet, alternating with a classical guitar and flute, played mournfully in the background as the litany unfolded.

Survivors, who wore white ribbons pinned to their chests, often broke off to add brief tributes. One fought to control himself as he condemned the "cowardly men" who killed his loved one.

A woman managed a smile as she called to her deceased husband Chuck, saying: "Until we meet again may God hold you in the palm of his hands."

At the Pentagon, thousands joined President George W. Bush and US Defense Secretary Robert Gates to dedicate the first September 11 memorial.

A Marine Corps bugler played taps from the roof where fire-fighters had unfurled an American flag while the building burned after the attack.

The sombre patriotism provided a rare moment of unity in a country less than two months from the end of an increasingly divisive presidential race.

But Obama set the tone, saying 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."

Because of continued delays in erecting the World Trade Centre's replacement and a Ground Zero memorial, the memory of the attacks remains literally an open wound.

Just this week, the first steel beams for a planned Freedom Tower were put in place at what is still a huge hole surrounded by cranes.

"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 said.