Two day major counter-terrorism exercise to take place in London from today

London's largest ever counter-terrorism exercise will take place today and tomorrow and will involve 1,000 police officers at various locations across the capital, as well as soldiers, emergency services and intelligence officers.

The exercise, codenamed Strong Tower, is not based on any specific intelligence but is part of a long-term strategy of planning and preparing for all possible types of terrorist attacks, the BBC reported.

Planning for the two day exercise started in January, and only a dozen people know the full script to ensure that the officers and other services participating face the maximum challenge.

While much of the exercise will be hidden from view, senior officers have cautioned that there may be times when the public may hear loud noises or see areas being cordoned off.

Scotland Yard will be posting updates about the exercise on Twitter, using the hashtag #999exercise.

BBC said the exercise is influenced by what happened during two incidents last year – the Charlie Hebdo attack in Paris and the café siege in Sydney.

It said Scotland Yard has stressed that the exercise does not mean that it has any specific intelligence that such an event would occur but it is planning for every conceivable type of incident.

"The exercise will be an on-going fast-paced terrorist situation and we will explore how we as London's agencies respond," Deputy Assistant Commissioner Maxine de Brunner, the exercise's director said.

"The exercise is at the extreme end of what might happen. We're doing this so that if the very worst happens we are prepared to do our very best. It's only through carrying out thorough exercises and challenging assumptions can we be confident in dealing with different scenarios.

"We have not put together an exercise of this magnitude before – officers will not have come across anything like this before. It is about making sure we have the capability and skills for anything that terrorists could throw at us."

The last counter-terrorism training exercise took place out of view around the country, the BBC said. The last comparable exercise was held in London in 2012 when the Metropolitan Police and others tested how they would respond to an attack on the London Underground involving a possible radiological or chemical device.