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".
"Every Olympic Games in recent times has had a significant military component to its security plan. "There are no specific threats to the Olympic Games but we're going to have very large numbers of foreign visitors in London. "The world's eyes will be upon us and we want to make absolute sure that we do everything to maintain the security and safety of the Games. "Ocean's presence on the Thames, I hope, will be a very great reassurance to people attending the Games. I don't think it's 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".
"What we need to do is make sure we practise against those high-end threats but they are not considered to be likely. "What I'm doing is testing my systems so I'm reassured that, should they become more likely, we can react. "One would want the world to know that we are taking security for the Olympics seriously."
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.
"The versatility and manoeuvrability of helicopters allows them to get to the scene of action very swiftly to identify and to divert any surface vessels that are heading up the Thames, to ascertain their intentions and if necessary take action."
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.