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The security services have been practising their tactics if terrorists mounted an attack in the UK. But what is the advice for the public?
On 2 December the UK authorities mounted a long-planned training exercise which simulated a terror attack on a shopping centre in the City of London.
But what should members of the public do if they were caught up in a real gun attack?
The governmen advice can be summarised in three words: run, hide, tell.
If possible run away, urging others to go with you.
If not possible, hide, preferably behind cover such as substantial brickwork or heavy reinforced walls. Be aware of exits and try not to get trapped.
If you are sheltering behind a door, lock it if possible or barricade it if not. Then move away from the door. Keep quiet and silence your phone.
If it is safe to do so, ring 999 and tell the police where and when you last saw the suspects.