A mock terrorist firearms attack is being staged in London today to test the emergency services' ability to respond to a Tunisian-style mass shooting.
The multi-agency counter-terrorism "major live play exercise" - which will end tomorrow afternoon - has been in the planning since January and will involve a number of locations, the Metropolitan Police said.
The Charlie Hebdo attack in Paris and the hostage-taking siege in Sydney had an "influence" on the test, according to deputy assistant commissioner and director of the exercise Maxine de Brunner.
She said it was the Met's view that it had never before put together an exercise of this magnitude and said it would be "very realistic".
She also said the Mumbai attacks of 2008 "and other attacks around the world" were thought through in the planning of the exercise.
But the Met said the test was in no way connected or prompted by the massacre in Tunisia last week and the essential theme of it was agreed before January.
Ms de Brunner said: "Sadly London is no stranger to terrorism and we, alongside our colleagues, have to deal with the tragic events of July 2005. Given the changing nature of the very real terrorism threat and events around the world - Paris and Australia - we need to constantly adapt our plans and prepare for new and emerging threats.
"What is vitally important to all the agencies involved who routinely work together, and to Londoners, is that when the very worst happens we are prepared to be the very best and we can understand how each other work and how we'll all operate together."
She added: "Exercising is a vital part of our preparedness to respond to any major incident in London. It helps us to ensure that we and all our partners are (as) ready as we can be for whatever might happen."
The exercise will see a test of the emergency service response to an attack at the disused Tube station at Aldwych.
It is developed by the Home Office and the Met and is part of a Home Office national counter-terrorism rolling exercise programme across the country.
Over the course of two days, the Met, emergency services, the Government and key London agencies will be "put through our paces to test our response to a major incident", Ms de Brunner said.
"What is particularly significant about this exercise is that throughout it there will be significant live play elements. That means us and our colleagues actually responding to an incident on the ground," she said.
"So the exercise scenario is designed to test command and control and co-ordination of a multi-site marauding terrorist attack."
The first responders will be challenged by mounting an operation to contain the area, evacuate the public, rescue and treat the wounded, manage a crime scene, and catch the people responsible.
Within the Met "every level" of the response will be tested from the first responders to the decision makers.
How agencies co-ordinate their response throughout the incident will be explored as well as how they link to government crisis decision making.
More than a thousand officers from the Met will participate, but policing levels across London will remain unaffected by the exercise.
"This exercise is focussed in the extreme end of what may happen," Ms de Brunner said, adding that it was not taking place in response to any specific intelligence.
She said some of the activity would be "highly visible and audible" to the public, as elements would be "noisy".
The vast majority of police officers involved have no idea about the venues or what is planned, and know onlythat they are taking part in an exercise.
Only around 12 people will know the fine details of the exercise in advance.
The aim is to get the "maximum learning" from it and there will be a full debrief.
Among those taking part are all of London's emergency services, Greater London Authority, Transport for London, Home Office, Cabinet Office, Ministry of Defence, the Department of Health and NHS England.
Surrey Street will be shut for the day in order to minimise disruption.