Police officers, intelligence official and soldiers in the United Kingdom are taking part in a large counter terrorism exercise.
It comes just days after 38 people, primarily British tourists were killed by a gunman in Tunisia.
Sir Bernard said: “With events like today we are committing around 1,000 people to exercise our plans and make sure that should the worst happen we are ready. And we will be.”
Scotland Yard began planning the simulation in January, which sees actors playing the role of wounded and panicking civilians. Altogether 1,000 people will take part in the exercise.