One officer confirmed they were not looking for a car bomb, but were checking buildings in the heart of London's financial centre for a smaller device. Some offices were being evacuated as many lunching workers were prevented from returning to their buildings.
The scare follows the explosion of a bomb under Hammersmith bridge in west London early yesterday morning.
More than 24 hours after the explosion, no group has admitted planting the bomb, amid speculation that it was the work of dissident Irish republicans.
Deputy assistant commissioner Alan Fry, the head of Scotland Yard's anti-terrorist branch, urged people to be vigilant in the wake of the attack. "I am asking people that if they see a suspect package to contact us immediately on 999."
He added there would be more high visibility policing on London's streets in the wake of the bomb and said measures would be taken to reassure the public.
