Emergency services acted out a scenario where terrorists had taken dozens of hostages on a boat on the River Thames earlier today.
More than 200 police officers have been joined by personnel from other agencies, including the London Ambulance Service, London Fire Brigade and the RNLI.
The exercise scenario deals with the hijacking of a river cruise boat, with dozens of hostages on board.
It is arguably the most challenging exercise UK armed police have been involved in, as they attempt to board a vessel in the middle of the country’s busiest river.
The exercise comes just a day after an attacker was shot dead after he tried to disarm a French soldier at Orly airport in Paris. Authorities there are treating it as a terrorist incident.
Police in London said it highlights the constantly evolving nature of the threat facing European cities and underlines the need to train here for many scenarios.
Scotland Yard Commander BJ Harrington, in charge of the Thames exercise, said: “We’ve seen attacks across the world, with tragic circumstances and in different guises. What we do is test and exercise all the time.
“This is just a natural progression into what is a busy waterway through the centre of the capital, to make sure that there is a good police response that works well in a marine environment.”
The Thames exercise is the latest in a regular series of armed training scenarios that police in several UK cities have been involved in over recent months, aimed at testing the effectiveness of the response here to a mass gun and bomb attack.
Scotland Yard said the decision to mount an operation on the River Thames was not in response to any particular threat.
The terror threat level in the UK remains at Severe, the second highest position, indicating a terrorist attack at some point is highly likely.