A major counter-terrorism exercise is being carried out in London for a second day to ensure emergency services are prepared.
A mock terrorist firearms attack is being staged in the capital to test the response to a Tunisian-style mass shooting.
Officers, emergency services, soldiers and intelligence officials are taking part in the operation.
Armed gunmen stalked the streets of central London today firing blank bullets as part of a police training exercise.
More than 1,000 police took place in the exercise - where gunmen simulated attacking commuters in the London Underground - and more than 12 other agencies and emergency workers.
Rags Martel reports.
Armed gunmen were seen stalking through the streets of central London today as part of the biggest ever police training exercise for a terrorist attack.
The masked men were seen firing guns in the air and throwing what appeared to be grenade-like devices inside the disused Aldwych station.
Preparations for the exercise began around six months ago and events such as the Charlie Hebdo attack in Paris and the hostage siege in Sydney influenced the test, police said, stressing it was not connected to the Tunisia attacks.
The two-day exercise will see a test of the emergency service response to an attack at the disused Tube station at Aldwych. The vast majority of police officers involved have no idea about the venues or what is planned.
Only around 12 people knew specific details of what was going to happen in advance.
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.
Rapid gunfire has been reported around the Strand, where police are in the midst of a major training exercise.
Sitting outside at Ibm southbank - can hear sounds like rapid gun fire across the river near aldwych direction !!
Earlier police said the exercise is to test the response by emergency services to the threat of a "marauding terrorist firearms attack in London."
Met Police chief Sir Bernard Hogan-Howe said that one of the objectives of today's terror training exercise in London is to "learn from the mistakes we're bound to make."
"Best we make them today in an environment which we don't have terrorists that make them when we do," he said.
Emergency services from over 14 London agencies are taking part in a major training exercise across the city.
There have been reports of rapid gun fire in the simulated terror attack from passers by in the Aldwych area.
Police have warned there will be some disruption across the city today as they hold a major terror training exercise.
More than 14 different organisations and agencies are taking part in the exercise, which will take place around the disused Aldwych underground station in Surrey Street, WC2.
Police, emergency services, intelligence agencies and transport staff and agencies are taking part in a training exercise to test London's robustness to the event of a terrorist firearms attack.
Exercise Strong Tower has been planned since January, and will take place at different site across the city - including at Aldwych station close to the strand.
A "very realistic" mock terrorist firearms attack is being staged in London today and tomorrow to test the emergency services' response.
The exercise has been in the planning since January and will involve a number of locations, the Metropolitan Police said.
Deputy assistant commissioner and director of the exercise Maxine de Brunner said the Charlie Hebdo attack in Paris and the hostage-taking siege in Sydney had an "influence" on the test.
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.
...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.
The Met said the test was in no way connected or prompted by the massacre in Tunisia last week.
More than a thousand officers from the Met will participate, but policing levels across London will remain unaffected by the exercise.
Some activity will be "highly visible and audible" to the public, as elements will be "noisy," the Met said.
The police commissioner will host a counter terrorism conference in London today to discuss the changing terrorist threat in Europe and elsewhere.
Sir Bernard Hogan Howe will be joined by more than 100 senior representatives from police and security forces, as well as partners from other countries recently affected by terrorist attacks.