Police, firefighters and various agencies are staging a major exercise to test their response to a terrorist attack.
The two-day Operation Strong Tower which began on Tuesday morning will test how they react to terrorists armed with firearms in central London.
The operation comes just days after the horror beach massacre in Tunisia but police say it had been planned for six months.
On Tuesday, the tactical response on the ground will be visible to media and passers-by outside the disused Aldwych underground station in Surrey Street which is closed for the day.
Deputy Assistant Commissioner Maxine de Brunner, who is the Exercise Director, said: "Testing and exercising is a really important way to ensure that our plans to respond to a major incident or terrorist attack are as good as they can be.
"We put huge effort into our planning and want to test how we all work together.
"The exercise scenario will test our tactical response including working with our emergency services partners, containing the situation, evacuating the injured and managing a crime scene.
"We've carefully planned the live play elements of this exercise to make sure that any disruption to the public is kept to a minimum."
As the exercises progresses it will test how senior decision makers manage the impact of the incident on responding agencies, an ongoing fast paced counter terrorism investigation and a range of issues that impact on life in the Capital.
DAC de Brunner added: "Sadly, London is no stranger to terrorism.
"Given the changing nature of the very real terrorism threat and events around the world we need to constantly adapt our plans and prepare for new or emerging threats.
"What is vitally important to all the agencies involved, who work together everyday, and to Londoners is that when the worst happens we are prepared to be the very best we can. We have sound plans and highly dedicated, brave staff that Londoners can be confident in."
The exercise will finish in the afternoon of Wednesday, 1 July.
Officers have been briefing local communities and businesses on the exercise and what they may see and hear.
A full debrief will be carried out after the exercise to make sure that any learning is properly captured.
This operation is part of the Government's regular counter terrorism exercises and jointly developed by the Metropolitan Police Service and the Home Office.
More than 14 different organisations and agencies are taking part.
Met Police Commissioner Sir Bernard Hogan-Howe said: "The reason we have exercises like today is because, obviously, we are concerned there are people planning terrorist events.
"We intend first of all to stop them from getting to attack. But should we not stop the terrorists in their planning, it’s essential we disrupt them in any of the attacks that may take place.
"With events like today we are committing around 1,000 people to exercise our plans and make sure that should the worst happen we are ready - and we will be."