This is what the BBC (BBC News24) is calling what is currently going on in London's Whitehall - the pictures they have been broadcasting show Whitehall completely closed to traffic, although a single-decker bus can be seen.
More as situation develops ...
12.20 - They are just now saying that security personnel are checking a 'supsicious package' on a double-decker bus, close to the end of Downing Street, but this bus is not visible on the screen Nothing on BBC website yet, though.
12.25 - Latest I am hearing is that it is thought that the security situation may be about to be cleared as the security alert is in its 'final stages'. Some official traffic now seems to be moving away from scene.
12.30 - Ordinary traffic now moving up Whitehall and it is being announced by the Police that the 'security alert is over'.