If you're around the station mid morning on Wednesday, you'll see people evacuated onto Surrey Street as part of the test on Wednesday. Other security work will be carried out through to Thursday.
Emergency services, cabinet ministers and London Underground staff will be acting as though it is August 8 and 9, predicted to be two of the busiest days in London during the Games. More than 2,500 people are expected to be involved in the simulation.
Met Police Assistant Commissioner Chris Allison, who is also the National Olympic Security Coordinator, said "This is part of a national exercise. The majority of stuff that the public will see, because clearly they will not be underground, will be the people coming out of the Tube system afterwards.
"It is for us testing our first responders capabilities and what we have learned to make sure that we have got that in place from 7/7 (the July 7 2005 bombings).
"A significant amount of work has been done in relation to those recommendations. At their heart, they're about further developing partnership working, and further developing the response that has to be put in place, not only by the three emergency services but also by Transport for London"
Assistant Commissioner Specialist Operations Cressida Dick, the National Director of Counter Terrorist Policing during the Games, called it “one of the extreme scenarios that we could face”.
She said: "This exercise and its scenario is not as a specific result of intelligence. The scenario has been planned for many many months. We will be doing our very best to prevent such an attack but it would only be right that we test our response to such an attack."
See The Guardian for more on this. It's great to see this exercise taking place and watch out on TV news this evening for coverage.