The Royal Navy's largest warship has sailed up the Thames as the military stepped up security preparations for the Olympics.
HMS Ocean berthed at Greenwich in east London this afternoon ahead of a security exercise named Olympic Guardian.
The ship will act as a launch pad for eight Lynx helicopters and a base for Royal Marine snipers.
Defence Secretary Philip Hammond, who was taken on board by a landing craft, denied the preparations were "over the top".
General Sir Nick Parker, in charge of the military's Olympics role, said the security exercise would prepare for the possibility of "extreme threats".
HMS Ocean was successfully negotiated through the Thames Barrier, before beginning a 180-degree turn at West India Docks.
Captain Andrew Betton said the ship would focus on air security over the weekend, with river operations taking place between Tuesday and Thursday next week.
Capt Betton said Ocean would provide vital assistance in stopping any potential threats on the river.
Lieutenant Mike Robertson from 661 Squadron, who will fly Army Lynx helicopters from HMS Ocean, said there were "a huge number of threats" that the military could potentially face.
His unit returned from Afghanistan only five months ago, and he described the transition to protecting London as "very challenging".
Also as part of the Olympic Guardian operation, soldiers took to the rooftops of residential areas in London, as part of the capital's security training exercise ahead of the Olympics.
Troops were seen on the roof of a building in the Bow Quarter development in east London, where surface-to-air missiles are set to be stationed despite opposition from residents.