Memorial services are being held to remember the 2,974 victims of the 9/11 terror attacks seven years after the atrocities.

Family members and students representing the countries that lost individuals have begun reading out the names of those who died, including 67 Britons.

Later, US presidential candidates Barack Obama and John McCain will visit Ground Zero in New York, the site of the Twin Towers.

On September 11, 2001, 19 terrorists took control of four planes and flew them into the North and South Towers of the World Trade Centre as well as the Pentagon, while a fourth plane crashed into a field near Shanksville in rural Pennsylvania.

Passengers and crew of United Airlines Flight 93 had attempted to retake control of the aircraft which the hijackers ultimately planned to fly into the US Capitol building in Washington DC.

In New York, Mayor Michael Bloomberg led the first of four silences to mark the moments the Towers were hit and when they fell.

He said of that day, at a solemn ceremony in New York: "It lives forever in our hearts and in our history, a tragedy that unites us in a common memory and a common story.

"We return this morning as New Yorkers, Americans and global citizens, remembering the innocent people from 95 nations and territories that lost their lives together that day."

Meanwhile, Prime Minister Gordon Brown has pledged the continuing "support of the British people for America" on the anniversary.

Mr Brown said he will be speaking to US President George W Bush later and urged people to remember all those who died in the attrocities.

At 8am, Robert Tuttle, the US ambassador to Britain, laid a wreath at a memorial near the US Embassy in Grosvenor Square, central London, to honour the British victims.

More than 200 free concerts have been organised across the world with simultaneous events on both sides of the Atlantic.

After performances featuring musicians who lost loved ones in the atrocity, audiences in London and New York will join the artists in a rendition of John Lennon and Paul McCartney's classic Let It Be.

London's concert will take place in Grosvenor Square at the same time as performers take to the stage in the British Memorial Garden in Hanover Square, New York.

In the UK capital, Coventry singer/songwriter Rob Halligan, whose father died in the World Trade Centre, will perform. Additionally, singer Kris Buckle, the London Welsh Chorale and South African opera singer Siphiwo Ntshebe are on the bill.

The annual concerts were the brainchild of New York businesswoman Hakuro Smith, who in 2002 decided to try to turn September 11 from a day of anguish into "a day of music to celebrate our universal humanity".

She said: "The September Concert is not a memorial event. It's an opportunity for people to come together once a year and be united in hopes for peace and celebrate our universal humanity. Music has the magical power to do that."

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