Armed police have played out the chaos of a "pleasure boat" terrorist attack on the Thames in a unique marine-based exercise to test the emergency response to gunmen.

A sightseeing vessel became the scene of a fierce mock-gun battle between armed officers and police volunteers posing as terrorists shortly after 11am on Sunday, close to the London Docklands area.

The exercise marks the first time the Metropolitan Police have enacted a live-action, water-borne exercise of its kind, with over 200 officers involved.