The Royal Navy's largest ship has sailed up the River Thames on Friday ahead of a major security exercise in preparation for the Olympic Games.
HMS Ocean has berthed at Greenwich in east London where it will act as a launch pad for eight Lynx helicopters and a base for Royal Marine snipers.
It is the final phase of the exercise named Olympic Guardian, which began earlier this week in Weymouth and in the airspace over the capital.
Smaller crafts and patrol boats will accompany the ship as part of the maritime security plan.
HMS Ocean sets sail for Greenwich
The snipers have extensive experience of shooting the engines of fast vessels in the Royal Navy's fight against the drug trade in the Caribbean.
Ocean has travelled up the Thames before attempting and successfully completed a complicated 180-degree turn as it reaches its destination.
The warship is 38 metres wide, and Captain Andrew Betton carefully guided it through "challenging" gaps of just 60 metres.
He said the ship would focus on air security over the weekend, with river operations taking place between Tuesday and Thursday next week.
"That will stretch all elements of Ocean's contribution to the security plan, and then I hope assure the security organisation that we are ready to provide the envelope required for the Olympic Games themselves," he said.
Capt Betton said the ship would provide vital assistance in stopping any potential threats on the river.
HMS Ocean Captain, Andrew Betton
"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," he added.
Lieutenant Mike Robertson from 661 Squadron, who will fly Army Lynx helicopters from 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".
Lt Robertson added: "I'm very excited as are all the squadron to be part of the Olympic Games.
"It's a once in a lifetime opportunity to be part of the greatest show on Earth, especially in your home country."
During the Games the ship will also accommodate 300 military personnel who are providing security at nearby Greenwich Park, and engage with Londoners by hosting public visits.
Last year it spent four months operating off the Libyan coast.
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Defence Secretary Philip Hammond was taken on board Ocean by a landing craft shortly after 9am.
He addressed the ship's company in the hangar before taking a tour of the vessel.
"There are no specific threats to the Olympic Games but we're going to have very large numbers of foreign visitors in London," he said.
"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."
General Sir Nick Parker, in charge of the military's Olympics role, said the security exercise would prepare for the possibility of "extreme threats".
He said: "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."
He added: "One would want the world to know that we are taking security for the Olympics seriously.
"The Metropolitan Police have asked for our support in certain key areas and we're taking that support extremely seriously."
The Mod's 'reassurance' strategy has caused bemusement amongst residents of a tower block in Bow, East London, who this week received leaflets informing them that high velocity surface-to-air missiles are to be deployed on top of their flats.
The government is not only expecting physical attacks on the capital during the games. Cabinet Office minister Francis Maude this week warned about the potential danger from cyber-attacks.