A two-day major counter-terrorism exercise involving police officers, soldiers, emergency services and intelligence officials started in London on Tuesday, the media reported.
London's largest counter-terrorism exercise - codenamed Strong Tower - involves over 1,000 officials at locations across the city until Wednesday, BBC reported.
The Metropolitan Police says the exercise was not based on any specific intelligence and was part of a long-term strategy of planning and preparing for all possible types of terrorist attack.
Deputy Assistant Commissioner Maxine de Brunner, the exercise director, said the aim was to test and challenge police and other agencies in an ambitious multi-site marauding terror attack, BBC said.
"This exercise will be an ongoing fast-paced terrorist situation and we will explore how we as London's agencies respond," de Brunner said.
"We have not put together an exercise of this magnitude before -- officers will not have come across anything like this before," he added.
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The last comparable exercise of this nature was held in London in 2012.
In that exercise, the Metropolitan Police and others tested how they would respond to an attack on the London Underground involving a possible radiological or chemical device.