If you live in London and don't like surprises, it might be best to stay home today.
The emergency services are doing a training exercise on the streets of London simulating a terrorist incident, and it is very realistic looking, as the videos below from near Aldgate station show.
Actors playing gunmen and victims can be seen shouting and screaming. Gunfire and explosions are also part of the drill.
The exercise, codenamed Strong Tower, is the largest ever terror attack drill undertaken by the Met. More than 1,000 police staff and other emergency responders will be training today to test how well they can assess dangers, treat the wounded, evacuate people and catch the perpetrators.
The police have taken pains to emphasis that the drill has been planned for months and has nothing to do with the deaths of British tourists at the hands of Isis in Tunisia last week.
The terror threat level in the UK remains at "severe" following Friday's three attacks in Tunisia, Kuwait and France.
Scotland Yard’s Twitter account @metpoliceuk is giving updates using the hashtag #999exercise.