Anti-terror police are to swoop on a shopping centre in a training exercise to test their response to a Paris or Brussels-type attack on civilians.
Armed with machine guns, officers from the North West Counter Terrorism Unit will drill their training techniques and counter-terror tactics during the exercise at the huge intu Trafford Centre shopping complex on the outskirts of Manchester, starting at midnight on Monday.
Around 800 people will be involved in the role-play exercise, including volunteers and fire and ambulance service personnel, with police wanting to make the "attack" as realistic as possible.
The exercise will stretch over three days, involving a second location on Merseyside.
Exact details have not been made public and are known only to a few senior officers, who will direct different scenarios such as possible hostage taking, mass casualties or suicide bombers, to test how the emergency services respond to "fluid" and developing situations.
Police stress there is no specific threat to the shopping centre and the exercise has been planned since last December, but it will give them and businesses the opportunity to test their emergency terror response.
The current UK terror threat level is described as severe, meaning an attack is highly likely, according to the security services.
The exercise, codenamed Exercise Winchester Accord, is the fourth major exercise in recent months, coming after previous ones in London, Glasgow and Essex.
The latest drill takes place while the shopping centre is closed to the public between midnight and 6am.
Assistant Chief Constable Rebekah Sutcliffe of Greater Manchester Police said: "This exercise is part of a national programme that has been planned extensively for five months.
"We have worked closely with intu Trafford Centre and other emergency services to test our response to a major terrorist incident in a public place, which for part of this exercise is a shopping centre.
"Residents in the area may hear loud noises and see emergency services activity around intu Trafford Centre during the exercise and I can reassure people that there is no cause for concern.
"However, I would still urge people to contact police if they do have any concerns or want to report anything and are not sure if it is linked to the exercise."
Any suspicious activity or behaviour can be reported to police via the confidential Anti-Terrorist Hotline by calling 0800 789 321. Extremist or terrorist content online can be reported at www.gov.uk/report-terrorism.