More than 60 people were hurt in the explosion. At least five people remain in critical condition and others have had to have limbs amputated.

Most of the casualties were taken to two of London's main hospitals, St Thomas's and University College Hospital, where major incidents were declared.

At least 80 people were hurt

St Thomas's said all its surgical teams were at full stretch. Other staff who were off duty had also volunteered their services.

Hospital spokesman Matt Tee said: "There are blast injuries, mainly to arms and legs, and there appear to be nails involved."

The Royal London Hospital, Whitechapel, sent its helicopter ambulance service with doctors and medical equipment to the scene, landing at Trafalgar Square.

Chelsea and Westminster and Guy's hospitals are also on standby to take casualties.

'Victims are mostly male'

At least 60 people have been treated at four different hospitals.

Some 21 have arrived at St Thomas's, two of them critical, and 15 at University College Hospital, three of them critical and four with serious injuries. Four "walking wounded" have been treated at the Royal London, and 20 at Guy's, all minor cases.

London Ambulance Service said there had been more than two amputations, but could not say how many.

Louise Bowden, Chief Nurse at University College Hospital, believed the injured were mostly male.

"Doctors seem to think it's a nail bomb incident,'' she said.

"When a bomb goes off, if it's near you it shreds flesh, it's pretty horrible really. People have serious lacerations, deep wounds and there are some serious burns. Some of them may lose limbs.''