Armed Police In Terror Drill
Armed police units took part in a counter-terrorism exercise in London. The drill simulated an attack by gunmen at a shopping centre in a building complex in the square mile. 
In the exercise, role players dressed as terrorists and armed with blank-firing assault rifles, moved through the building, firing indiscriminately. They also also carried simulated explosive devices. Dummies littered the hallways, simulating downed civilians.
Police units involved in the drill included British Transport Police (BTP) and the Met's SCO19. Police in Armed Response Vehicles (ARVs) were first on the scene, followed by SCO19 Counter Terrorist Specialist Firearms Officers (CTSFO). The officers stormed the complex, going room to room until it was clear.
The exercise shows how Police firearms units have adapted their tactics to face new threats. While the exercise may bring recent attacks in Paris to mind, it was planned long before that event. It follows a similar exercise in June which simulated an attack by gunmen at an underground station. Yesterday's exercise, however, was the first time that officials have talked about the new tactical paradigm.
Metropolitan Police Assistant Commissioner Patricia Gallan, head of Scotland Yard's Specialist Crime and Operations, said that armed police may have to "go forward" during an attack, including moving past civilian casualties and injured colleagues, in order to quickly confront and neutralise the terror threat. 
This is a change to previous tactics that emphasised containment and negotiation, followed by a carefully-planned intervention.
The new tactics were adopted in the wake of the 2008 attacks in Mumbai, which indicated that an immediate and decisive response is needed in order to save civilian lives.
Securing the area as quickly as possible hopefully allows medics to treat survivors in the so-called 'golden hour' - the 60 minutes following a traumatic injury during which treatment has the best chance of preventing fatalities.
Security planners have been preparing for a Mumbai-style event in London and other major UK cities for several years.
There are around 2000 armed police officers in London. Armed Response Vehicle (ARV) constantly patrol the streets and are likely to be the first armed units to arrive at an incident.
The Met can also mobilise SCO19 Tactical Support Teams (TST) and the highly-trained CTSFO branch.
There is also an improved relationship between the police and the intelligence services which serves to better prevent attacks from taking place.
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More info / further reading
- 1. Heavily armed police train for Paris-style terror attack on the streets of London
- 2. Terror attack might prompt 'go forward' tactic, says Met