Ceremonies marking the 10th anniversary of the 11 September attacks in the US have taken place around the world.

In New York the name of every person killed in al-Qaeda's attack was read aloud in a nearly five-hour-long ceremony where the World Trade Center's twin towers stood.

There were smaller ceremonies in Shanksville, Pennsylvania, and at the Pentagon outside Washington, the other sites where 19 men from the militant group al-Qaeda crashed hijacked planes ten years ago.

The attacks led US forces to invade Afghanistan to topple the Taliban rulers who had harboured al-Qaeda leader Osama bin Laden. Washington also began a "war on terror" that ousted Iraq's Saddam Hussein and that persists on several fronts to this day.

"Ten years have passed since a perfect blue sky morning turned into the blackest of nights," New York Mayor Michael Bloomberg said at New York's Ground Zero.

"Since then, we've lived in sunshine and in shadow, and although we can never unsee what happened here, we can also see that children who lost their parents have grown into young adults, grandchildren have been born and good works and public service have taken root to honour those we loved and lost."

With security tight and no traffic, there was an eerie silence in New York where the 110-storey skyscrapers collapsed a decade ago, sending a noxious cloud over lower Manhattan.

For the first time, relatives saw the just-finished memorial and touched the stone where the names of their dead loved ones were etched.

The New York memorial includes two plazas in the shape of the footprints of the twin towers with cascading 9.1m waterfalls.

Around the perimeters of pools in the centre of each plaza are the names of the victims of the 11 September attacks and an earlier 1993 attack at the trade centre.

President Barack Obama visited the North Memorial Pool, which sits in the footprint of the north tower.

He walked around it hand-in-hand with first lady Michelle Obama, and with former President George W Bush and his wife, Laura.

The president touched the etched names of the dead before he greeted some family members.

President Obama, who was set to visit all three attack sites, read from Psalm 46 in New York: "God is our refuge and strength."

At the Pentagon ceremony, Vice President Joe Biden said, "Al-Qaeda and Bin Laden never imagined that the 3,000 people who lost their lives that day would inspire 3 million to put on the uniform, and harden the resolve of 300 million Americans."

Police in New York and Washington were on high alert against a "credible but unconfirmed" threat of an al-Qaeda plot to attack the United States again on the 10th anniversary.

At Shanksville, President Obama laid a wreath where a plane crashed after passengers overwhelmed hijackers intent of hitting the White House or US Capitol.

Chants of "USA, USA" broke out from the crowd, gathered at the foot of a grassy hill.

The Obamas talked at length with family members. "Thank you for keeping us safe," one man yelled out to him.

Pennsylvania Governor Tom Corbett said the passengers on Flight 93 "set a new standard for American bravery."

Irish events to mark anniversary

As part of worldwide ceremonies to mark the tenth anniversary of the 11 September attacks, a multi-faith commemoration service took place this morning at the Dublin Fire Brigade Training Centre.

President Mary McAleese, US Ambassador Dan Rooney and Dublin Lord Mayor Andrew Montague attended the mass.

This afternoon, a ceremony of reflection and peace, hosted by Ambassador Rooney at the RDS, was attended by President Mary McAleese and Taoiseach Enda Kenny.

At 4pm, Ireland took part in a "Global Sing for Peace".

"Ireland Remembers" at the Convention Centre in Dublin was to include a programme of poignant and moving music, and incorporate spoken reflections.

A public ceremony to mark the tenth anniversary of the 11 September attacks also took place at Belfast City Hall.

The event was hosted by the city's Sinn Féin Lord Mayor, Niall Ó Donnghaile, and attended by the US Consul to Belfast.