An event like this is just begging for crowds of people lining the streets, ignoring the evacuation orders and carrying banners saying: "Forget the symptom, remove the cause" and "No justice, no peace" etc...

 http://news.bbc.co.uk/2/hi/uk_news/politics/3042134.stm

"US-UK plan terror rehearsal
A major simulated terrorist attack on both sides of the Atlantic at the same time is being planned in the effort to prepare against terror threats, it has emerged.

A chemical attack could be simulated on the Tube this autumn
terrorist attack on both sides of the Atlantic at the same time is being planned in the effort to prepare against terror threats, it has emerged.
A chemical attack could be simulated on the Tube this autumn
terrorist attack on both sides of the Atlantic at the same time is being planned in the effort to prepare against terror threats, it has emerged.

The exercise is expected to take place in 2005 and will involve senior government figures as well as emergency workers.

It will follow a huge operation planned for this autumn which will rehearse emergency procedures for a chemical, biological or nuclear attack on the London Underground. "