A large-scale military exercise to test security preparations ahead of the 2012 London Olympics is underway in the U.K. capital, as the Royal Navy's largest attack ship squeezes up the Thames River.

HMS Ocean, an amphibious assault ship designed to deliver troops via helicopter and landing craft, will be moored in the river to protect Olympic sites during the Summer Games, which take place between July 27 and August 12.

Exercise Olympic Garden, which runs between May 2 and May 10, will see a combination of army, navy and air force activity in southeast England.

“Whilst there is no specific threat to the Games, we have to be ready to assist in delivering a safe and secure Olympics for all to enjoy," Defence Secretary Phillip Hammond said in a release.

The military has also designated a number of areas for ground-based air defence missiles, including two atop buildings in east London as well as four others in outlying areas, though so far the placements aren't carrying any live weaponry, CBC's Susan Ormiston reported.

A member of the Royal Navy watches over the bow of HMS Ocean in the Thames River. (Susan Ormiston/CBC)

The U.K. government will decide whether to arm those sites after the conclusion of Exercise Olympic Garden, likely within the month, Ormiston said from London.

Those placements, along with the exercise as a whole, have attracted media attention and sparked a degree of concern among city residents.

"It has certainly been a talking point, a water-cooler issue," Ormiston said.

The BBC reported that the Stop the War Coalition has criticized the activity as "unacceptable," saying the military presence will create unnecessary fear.

The U.K. military has warned there will be an increased and audible presence of fighter jets and helicopters over the capital during the exercises, particularly on Saturday and Sunday.

Ormiston said the military has taken a "very public approach," taking members of the media to specific events in an effort to show Londoners, and the world, that military planners are ready for potential threats.

She said officials have told residents that the after the conclusion of the exercises the military units are supposed to slip into the background.