An increased number of police will be on duty across the country in the coming days, with MI5 stepping up covert operations in support of regular forces.
With the terror threat to the UK remaining at “Severe”, major cities in particular will see more uniformed and armed officers on the beat.
In London, security arrangements at Westminster are to be urgently reviewed.
An increased number of police will be on duty across the country in the coming days
Defence Secretary Sir Michael Fallon said the review would look at whether current security arrangements are “adequate” and whether police at the front gates should be armed.
The important thing is we mustn’t cower, we mustn’t allow the terrorists to succeed and to destroy our way of life, or to divide communities
Sadiq Khan – London mayor
But he said it was neither possible nor desirable to seal Parliament off from the public.
He said: “The important thing is we mustn’t cower, we mustn’t allow the terrorists to succeed and to destroy our way of life, or to divide communities.”
Police directing traffic on Westminster bridge
Mark Rowley, the Met’s senior anti-terror officer, confirmed that security was being increased.
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He said: “As people are out and about on the streets of the Capital this morning they will see more police officers on duty – armed and unarmed – and a mix of British Transport Police and the City of London.
“We have cancelled some leave and increased some duty hours and are working to make sure that we can out in force to reassure the public. We must not allow terrorists to create discord, distrust and fear.
“The police stand with all communities – later today a meeting of faith leaders will be held here at New Scotland Yard.
"Whilst our work to investigate and understand what happened yesterday continues with vigour – we must also reflect.”
He added: “I want to thank the public for their support and all their good wishes – I know it is appreciated by all those men and women who are out there today protecting us.
London Terror Attack: Latest Pictures Thu, March 23, 2017
4 dead and 40 injured in Westminster terror attack
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Conservative MP Tobias Ellwood (centre) helps emergency services attend to a police officer outside the Palace of Westminster, London
“Finally, I would also like to ask the public for their continued help and continued vigilance and if you see anything that causes you concern or raises your suspicions do not hesitate to call us – 0800 789 321 – or in an emergency 999.”
Intelligence agencies also revealed their operations would now be stepped up, following Wednesday’s attack.
The head of MI5 condemned the Westminster killings as “appalling and disgusting”.
But Andrew Parker, director-general of the security service, also confirmed that its operational response has been “fully mobilised”.
Armed police officer near Houses of Parliament
He said in a statement: “The thoughts of the men and women of MI5 are with the families of those killed in Westminster yesterday, and with the other innocent people injured in this appalling and disgusting attack.
“We stand shoulder to shoulder with our police colleagues, grieving at their loss while also applauding the professional excellence of their response.
“As Acting Deputy Commissioner (Mark) Rowley said, the security community prepares for such awful events – while working tirelessly to prevent them.
“The MI5 operational response is fully mobilised in support of the police.”