If terror came to London: Video shows what police would do if UK's capital was hit by a major terrorist attack

Police have been acting out a terror attack on the streets of London to test the response of the emergency services in the event of a Tunisia or Charlie Hebdo-style marauding terrorist threat to the capital.

Scotland Yard has been using the Twitter hashtag #999Exercise to provide updates during the two-day training, which is taking place at various locations across London.

David Cameron said the exercise had been planned following the terrorist attack at the offices of the Charlie Hebdo magazine in January, when Paris was gripped by three days of horror as gunmen went on the loose.

Last week's massacre by an Islamic extremist on a beach resort in Tunisia, which left about 30 British holidaymakers dead, has reiterated the threat that these random, marauding types of terror attacks pose to the UK.

The exercise is involving more than 1,000 police officers, including 130 counter-terrorism specialist firearms officers, who make up the elite unit of the operation.