Police have advised the public that the best course of action if they should be caught up in a terrorist gun attack is to run and hide.
The guidance comes in a four-minute film which sets out steps to follow if the UK is hit by a firearms or weapons strike.
Security services have been on high alert since the massacre in Paris last month and the terror threat level in Britain currently stands at severe - meaning an attack is seen as highly likely.
Police are working on hundreds of investigations and making an arrest a day.
The video says people should run if they hear gunshots as long as it will not put them in greater danger.
"Insist others come with you, but don't let their indecision slow you down," the video says.
"Consider your route as you leave. Will it place you in the line of fire? Is it safer to wait for the attacker to move away before you continue?"
If it is not possible to move to safety, then people are advised to hide.
They should consider their exits and escape routes when choosing a hiding place, avoiding dead ends and bottlenecks.
Mobile phones should be switched to silent and vibrate turned off, the video says, adding: "The best hiding place with protection from gunfire will have a substantial physical barrier between you and the attacker."
Those able to evacuate should get as far away from the danger area as possible and call the police.
The film says: "When the police arrive they will be armed. The police may be unable to distinguish you from the attacker.
"They may treat you firmly. Do everything they tell you to do. Don't make any sudden movements or gestures that may be perceived as a threat."
Police said the advice has already been issued to thousands of people during security training sessions but it is now being rolled out more widely.
Mark Rowley, the country's most senior counter-terrorism officer, said: "Everyone's aware of the terrorist challenges across the world and there have been some awful attacks.
"It's our view that this advice should be rolled out to the public so in the tragic event that anyone gets caught up in a rolling firearms or weapons attack they are better informed and better advised to protect themselves."
Deputy Assistant Commander Neil Basu said the public should not be alarmed by the release of the video.
"While the general level of threat to the UK is severe, the probability of being caught up in a firearms or weapons attack is very, very small," he said.
"However, it is important the public know what to do in the event of getting caught up in such an incident."