LONDON  - HMS Ocean, Britain’s largest warship, sailed up the River Thames into London on Friday to take part in a nine-day training exercise testing responses to any attack on the forthcoming Olympic Games.

The amphibious assault ship squeezed narrowly through the river’s flood barrier on its way to moor at Greenwich, completing the military presence for Exercise Olympic Guardian, which started on May 2. Four Royal Navy and four army Lynx helicopters will operate from the 21,500-tonne ship, the defence ministry said.

HMS Ocean can house more than 1,100 people plus torpedoes, guns and radar, while small craft operated by marines are also present on the river. Typhoon fighter jets and surface-to-air missiles are already stationed around the capital, and Navy flagship HMS Bulwark is moored off the south coast for the exercise ahead of the Games, which run from from July 27 to August 12. HMS Ocean was trending nationally on Twitter as Londoners reacted to the unusual traffic on the river. Last year the warship spent four months operating off the Libyan coast.