Defence Secretary Philip Hammond has denied that sailing the Royal Navy's largest ship up the River Thames was "over the top" in preparation for the Olympic Games.
HMS Ocean berthed at Greenwich in east London ahead of a major security exercise named Olympic Guardian.
The ship will act as a launch pad for eight Lynx helicopters and a base for Royal Marine snipers.
Mr Hammond, who was taken on board by a landing craft, said: "I don't think it's over the top. Every Olympic Games in recent times has had a significant military component to its security plan."
The Conservative Cabinet minister, who joked with photographers after he tripped on his way to address the ship's company, added that Londoners and foreign visitors would be "reassured" by the military presence.
"There are no specific threats to the Games at the moment and we hope it stays that way, but if any threats do emerge I hope that the sight of these assets - this ship in particular - will reassure them that we will deal with any security threats in a determined and resilient way," he said.
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."
The sailing of Ocean to Greenwich was the final phase of Olympic Guardian, which began earlier this week in Weymouth and in the airspace over the capital.
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," he added.