A woman has received minor injuries in a "small" explosion at an office building in central London.
The area around Victoria Street was initially cordoned off
Scotland Yard said a letter bomb was found in a post room of the Capita building in Victoria Street, Victoria.
London Ambulance said a female employee had been taken to hospital for treatment to her injuries after opening the package - a small Jiffy bag.
Bomb disposal experts were at the scene and Scotland Yard's anti-terrorism command was investigating.
The scene is 200 yards from Scotland Yard's headquarters. Staff have now been allowed back into the Capita building which was evacuated after the explosion.
Chief Supt Ian Thomas, of the Metropolitan Police, speaking at the scene, said: "At about 9.40am emergency services were called to the offices behind us following reports of a small explosion.
"We are still investigating the cause of that explosion and the Met Counter Terrorism Command are looking at all the issues involved.
"One female employee was injured and taken to a local hospital and while this is a very traumatic incident for her, her injuries are fairly minor."
A Metropolitan Police spokesman said the counter-terrorism group was leading the inquiry "as a matter of procedure for an incident involving an explosion of this nature".
The Capita Group is a leading business out-sourcing company which runs London's congestion charge system among other services.
All six floors of the office were evacuated.
A spokesman for the company said: "Capita can confirm that a small incident took place at its Victoria street office this morning involving an incendiary device.
"The police have asked us to make no further comment at this time as their investigation is proceeding."
Ben Hetchin, an eye-witness, said: "At first we thought, presumed, it was a fire alert, as we often get, but then a woman was led out with her hands bandaged like a boxing glove, totally covered.
"She looked shocked and shaken, nervous. They then ushered us away behind the police cordon and two firemen said that it was a letter bomb on Victoria Street."
The location of the blast is less than a mile from the Houses of Parliament, Victoria Station and Westminster Abbey.