The Olympic flag is being raised above Downing Street, a week before the Games begin.
The five coloured rings set against a white background will be hoisted above No 10 as the countdown to the summer spectacle enters its final week.
It will be replaced by the Paralympic flag for the duration of the Paralympic Games from August 29 to September 9.
A No 10 spokesman said: "The 2012 London Olympic and Paralympic Games provide the greatest opportunity to showcase to the world all that is great about the UK, and Downing Street will be proudly displaying the Olympic flag from today and the Paralympic flag next month.
"These coming weeks are going to be like no others - they will be about making the most of our country, being everything we can be and being at the centre of the world's attention. Let's seize this opportunity and make the Olympic and Paralympic games a truly amazing event that the whole country can get behind."
The flag, which will also be flown from Whitehall departments, will fly above Prime Minister David Cameron's London home for the duration of the Games, which officially begin next Friday and run until the closing ceremony on August 12.
The International Olympic Committee, the London Organising Committee of the Olympic and Paralympic Games and Westminster Council granted permission for the flag to be hoisted from the No 10 flagpole.
Meanwhile, a Royal Marine Commando will abseil with the Olympic flame into the Tower of London.
Royal Marine Martyn Williams will arrive at Tower Wharf in a Royal Navy Sea King helicopter, and will undertake the 180ft abseil from the aircraft. The torch will then be carried around the Tower by an athlete and a Tower of London-nominated torchbearer.
It will be welcomed by Mayor Boris Johnson and Lutfur Rahman, the mayor of Olympics host borough Tower Hamlets. The flame will then be handed over to Lord Dannatt, Constable of the Tower of London. The torch will spend the night under guard at the 11th century Tower.