A knife-wielding terrorist rampaging through York Racecourse and suicide bombers targeting London's transport systems were part of two terrorism exercises held to assess the UK's emergency response systems.
Surprised onlookers were met with armed police and ambulances attending staged terrorism incidents where actors were covered in fake blood and injuries.
The events were held to test the UK’s response systems after emergency services were criticised following the Manchester Arena terrorist attack.
Suicide bomber Salman Abedi killed 22 people and injured more than 1,000 others when he detonated a bomb in his backpack at the end of an Ariana Grande concert in May 2017.
An inquiry into the attack made a number of recommendations, which included calling for emergency responders to become more effective and improve how they work together.
During the mock emergency, known as the Exercise Spring Resolve, the ability to stop attacks, save lives and effectively manage a series of surprise violent incidents across different regions were tested.
In York, the staged terrorist attack took place at the racecourse, where onlookers saw people stretchered out with fake injuries.
A person wearing a backpack and waving a knife stood outside the venue as armed police approached.
Security Minister Tom Tugendhat said the exercises were critical to ensuring that all agencies are prepared for terrorism incidents.
"The first duty of government is to protect the British people. Exercises like Spring Resolve are critical to ensure all agencies and departments are prepared for any type of attack," he said.
"I would like to thank all our frontline services for their diligent and brave work to protect and safeguard our country and people in the face of complex threats.
"The exercise scenario involved a mixture of live and notional play and included multiple attacks in rapid succession in London, designed to test fast and effective multi-agency response, communication and co-ordination. Attack locations included the transport network and other publicly accessible locations.
"The scenario incorporated a further no-notice attack at a publicly accessible location close to York city centre on the second day, again designed to test and exercise effective multi-agency response and recovery arrangements.
"The exercise tested communication flows from attack scenes, through strategic command centres and directly into central government, with Cobra meetings being held on both days, government ministers and senior officials also being exercised."
The National Counter-Terrorism Exercise, which was planned and co-ordinated by the Home Office, was staged on March 14 and 15 at various regional sites.
It involved the British Transport Police, Metropolitan Police, North Yorkshire Police, Counter-Terrorism Policing North-East, London Fire Brigade, London Ambulance Service, North Yorkshire Fire and Rescue Service and Yorkshire Ambulance Service.
The two-day exercise is part of the regular counter-terrorism testing programme that takes place across the UK.
“We know how vital it is that everyone involved in the response to a terrorist attack is fully prepared so that should the worst happen, together we are able to provide the best possible response," said Deputy Assistant Commissioner Laurence Taylor of Counter Terrorism Policing.
“Ensuring our plans are fit for purpose will ultimately help to save lives, and testing them regularly is crucial to ensuring police officers and our partners right across the UK understand their roles.”