Here is a look at Olympics staged by London - the only city to host the Games three times:
Athens has also held Games three times but its 1906 "intermediary" games were not considered official.
The Games were awarded to London at short notice when the Italian government decided it could not afford to host them following a volcanic eruption on Mount Vesuvius in 1906.
They lasted nearly six months after starting on April 27, 1908 and ending on October 21, 1908.
The first new purpose-built stadium was constructed at a cost of 60,000 pounds ($94,300)at Shepherds Bush in the west of London and known as White City. For the first time athletes marched into the stadium at the opening ceremony. The Games cost an additional 15,000 pounds.
1908 marked the first appearance of diving and hockey. Figure skating also appeared but was moved to the Winter Olympics in 1924.
The most dramatic event was the disqualification of Italian marathon runner Dorando Pietri, who arrived at the stadium to huge roars from the crowd but was so exhausted he went round the track the wrong way.
He then collapsed and was carried over the line by officials. American John Hayes crossed the line unaided and promptly protested the Italian's win and the protest was upheld.
The United States won 47 medals at the Games but Britain won 146.
The 1908 Games took receipts of some 21,000 pounds from total costs of around 75,000 pounds.
The 1944 Games had been scheduled for London but were cancelled as they had been in 1940 due to World War Two.
In 1948 the Games still attracted 59 nations and 4,104 competitors but Germany and Japan were not allowed to compete because of the their part in the war.
It was also the first ever Games to be shown on television, although very few people in Britain actually owned a set.
As the war had cost Britain 7 billion pounds, the 1948 Games were named the Austerity Games but still cost around 730,000 pounds which included the cost of housing, feeding and transporting the teams while in England. Receipts were 760,000 pounds.
In 1948 large parts of London lay in ruins following air raids during the war and rubble strewn streets were still a common site. Food rationing remained in force and building materials were in short supply.
Contingency plans were made and it was decided the Games would be held with no new venues or athletes' village being built. Competitors stayed at Royal Air Force camps, educational establishments and nursing homes. Prisoner of War camps were also used.
Vehicle headlights had to be used in the javelin event as there were no floodlights in Wembley Stadium.
The famous London Red Buses were used and military vehicles, many driven by women volunteers, were also used to ferry spectators and athletes alike to the venues.
An Arab boycott of the Games was averted when the IOC ruled that the new nation of Israel was not yet a member of the IOC and so therefore could not compete. Some countries competing for the first time included Burma, Lebanon, Panama and Venezuela.
Highlights included American athlete Bob Mathias, a 17 year-old who won gold in the decathlon only four months after taking up the sport.
The standout performer of the Games was undoubtedly Fanny Blankers-Koen, the Dutch sprinter. The housewife from the won four gold medals including the 80 metre hurdles, 100 sprint and the 200 sprint. She also ran the anchor leg in the 4x100 relay, guiding the Dutch to victory.
Britain won the right to stage the Games in 2005, seeing off strong competition from rival European cities Paris and Madrid.
Britain earmarked more than 9 billion pounds of public funding to build new facilities and provide security for the July 27-August 12 Games based in east London.
London organisers had a separate 2 billion pound budget for staging the Games, most of it from private sources.
The government has touted the Olympics as an opportunity to showcase business and tourism. Prime Minister David Cameron this month suggested the Games would generate 13 billion pounds ($20 billion) over four years.
The Olympics are being held at a time of deep spending cuts by the Conservative-led government and for the first time are being planned hand-in-hand with the long-term improvement of the area. The Olympic Park is located in the Lower Lea Valley and is the same size as 357 football pitches.
London is also hosting the Paralympics for disabled competitors from Aug 29-Sept 9.
26 Olympic sports 20 Paralympic sports
34 venues 21 venues
10,500 Olympic athletes. 4,200 athletes