US police have foiled an attempt to blow up a car at an annual Christmas tree lighting ceremony yesterday in Portland, Oregon.

The Justice Department identified the chief suspect as 19-year-old Mohamed Osman Mohamud, a naturalised US citizen of Somali descent.

Mr Mohamud was arrested after he attempted to detonate what he believed to be an explosives-laden van that was parked near the tree lighting ceremony in Portland's Pioneer Courthouse Square.

However, justice department officials said the device was faulty and the public was never in any danger.

'The threat was very real. Our investigation shows that Mohamud was absolutely committed to carrying out an attack on a very grand scale,' said Arthur Balizan, a special agent in charge of the FBI in Oregon.

'At the same time, I want to reassure the people of this community that, at every turn, we denied him the ability to actually carry out the attack.'