Police officers, soldiers, emergency services and intelligence officials are taking part in London’s largest counter-terrorism exercise to date.
The simulation of a terror attack has been six months in the planning.
The exercise – codenamed Strong Tower – involves 1,000 police officers at locations across the capital until Wednesday afternoon.
Metropolitan Police Commissioner Sir Bernard Hogan-Howe said it would test responses to extremists using firearms.
It comes days after 38 people were killed by a gunman in Tunisia, the majority of them British holidaymakers.
The Metropolitan Police says this week’s exercise in London is not based on any specific intelligence and is part of a long-term strategy of planning and preparing for all possible types of terror attack.