Norfolk police have stationed armed police in Norwich and Great Yarmouth as part of a county-wide operation to reassure the public following the Manchester terror attack.

Eastern Daily Press: Chief Constable Simon Bailey at Norfolk Constabulary Headquarters, Wymondham. Photo: Steve AdamsChief Constable Simon Bailey at Norfolk Constabulary Headquarters, Wymondham. Photo: Steve Adams (Image: Copyright Archant Norfolk 2015)

Norfolk police have told people they will be seeing an increased police presence, including armed units, at key locations around the county.

Some of those locations include Norwich Castle Mall and Great Yarmouth seafront.

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Officers are being deployed to reassure the public and will be out of their armed response vehicles.

In Great Yarmouth regular officers have increased patrols and in a rare move a police unit will be station at Yarmouth racecourse for today's races.

Roger Wiltshire, chief supt and head of Great Yarmouth Borough Police, said: 'There is no intelligence to say there is a threat to this area.

'But we will be carrying out patrols for reassurance.'

Armed police were also scene patrolling the Castle Mall shopping centre in Norwich and Robert Bradley, centre manager, confirmed their own high threat security measures are in place.

'In response to the heightened security level in the UK, police have stepped up presence in all public places such as shopping centres and city centre retail areas.

'Castle Mall works closely with the police and Norwich city centre to place our shoppers' security and wellbeing as highest priority. We have also put in place our own high threat security procedures to ensure our shoppers' continued safety.'

Theresa May raised the threat level to its highest ever rating last night, indicating that a further attack is imminent, after 22 were killed in a terror attack at an Ariana Grande concert in Manchester.

In a live televised statement from Downing Street, the prime minister said the terror threat level would be increased to 'critical', meaning an attack was expected imminently as it was possible a 'wider group of individuals' could have been involved in the Manchester Arena attack rather than just suicide bomber Salman Abedi.

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Chief Constable of Norfolk Constabulary Simon Bailey said: 'There is no specific intelligence relating to Norfolk however the public would expect us to respond accordingly to this raised threat level.

'As a result members of the public will see an increased policing presence at key locations – primarily those with high-footfall and we are reviewing key events over the coming weeks.

'Military personnel are not currently being deployed – but the threat assessment is continuously under review.'

MORE: UK terror threat raised to critical after bombing at Ariana Grande concert at Manchester Arena left 22 dead

'I would ask the public to remain calm but alert – if you see anything which causes you concern, then call the police immediately. Look out for anything that seems out of place, we would rather investigate concerns which come to nothing, rather than hear an incident could have been prevented.'

Critical threat level

What does that mean?

That authorities believe an attack may be 'imminent'.

Is this an unusual step?

Yes - though not unprecedented. It is the first time the assessment has been placed at the highest level for a decade.

It has only been at critical twice in nearly 11 years, in August 2006 and June 2007. On both occasions the assessment was lowered after a few days.

At what level had it been prior to this announcement?

Severe - the second highest - which indicates an attack was deemed to be 'highly likely'.

How long had the threat level been at severe before this change?

Nearly three years - it was raised from substantial in August 2014 after the rise of Islamic State.

How has the threat changed over the years?

The level was first made public in 2006. Since then, it has most often been at severe, and not been lower than substantial (an attack is a strong possibility).

How many levels are there?

Five. The other two levels are moderate (an attack is possible but not likely), and low (an attack is unlikely).

Who decides the threat level?

An organisation called the Joint Terrorism Analysis Centre. It was established in June 2003 and is based at MI5's London headquarters.

JTAC comprises representatives from 16 government departments and agencies.

How is a judgment reached on the threat level?

A number of factors may be taken into account, including available intelligence, terrorist capability, terrorist intentions and timescale.

• Latest guidance on recognising the terrorist threat and remaining vigilant can be accessed via

• To report any suspicious activity or behaviour call 0800 789 321 or in an emergency dial 999.