While security at such sites as Buckingham Palace, the Houses of Parliament and London's Canary Wharf has been stepped up, efforts to counter terror attacks are by no means as visible in the UK as they presently are in United States.

BBC: Met chief learns NY terror lessons

London's mayor and the 25-seat GLA will first take office in Marsham Street, close to the Houses of Parliament in central London before eventually moving to a new building near Tower Bridge.

BBC: Voters go to the polls

The Palace of Westminster, also known as the Houses of Parliament, is a complex of buildings in London.

UNESCO: Culture

Along with key landmarks such as Big Ben, the Houses of Parliament, and The London Eye going dark, the quadrocopters will turn off their glow in support of Earth Hour.

ENGADGET: Quadrocopter fleet stuns Londoners with giant hovering Star Trek logo (video)

Horrified onlookers watched as the helicopter hit the crane amid low cloud and fog near Vauxhall station, south of the river and not far from landmarks such as the London Eye and the Houses of Parliament, and came plunging to the ground.

CNN: London helicopter crash prompts safety questions

They may inflict "mass casualties" and constitute a "severe" threat to the Government Security Zone (including the Houses of Parliament and key executive offices) in the heart of London.

CENTERFORSECURITYPOLICY: Yes, they can

The London Eye, a giant Ferris wheel almost opposite the Houses of Parliament, will eventually be an easy walk from the arts centre at the South Bank.

ECONOMIST: The Thames

Deighan meantime had fled to London, where he was caught on CCTV at a McDonalds near the Houses of Parliament.

BBC: Andrew Deighan guilty of murdering Matthew Martin