Organizers say the ceremony — scheduled to last two hours, 45 minutes — is being broadcast to more than 300 million viewers around the world.
The show is set to include musical performances by the Spice Girls, The Who, George Michael, Annie Lennox, the Pet Shot Boys, One Direction, Ray Davies and the surviving members of Queen.
Thousands of athletes will be marching into the stadium and the Olympic flag will be handed over to Brazilian organizers of the 2016 Olympics in Rio de Janeiro. There will be closing speeches by IOC President Jacques Rogge and chief London organizer Sebastian Coe before the flame is extinguished.
Soccer star Christine Sinclair will carry Canada's flag at the closing ceremony of the London Olympics.
Sinclair was chosen Sunday to lead the Canadian team into London's Olympic Stadium for the ceremony.
"This is a huge honour and I wish I could have my teammates right here with me," said Sinclair. "We want the next generation of young girls to dream of being in the Olympics and getting a medal around their necks. If we can instill in them just the belief that this can happen, then we have done our jobs."
Sinclair captained the Canadian women's soccer team to a bronze medal — the first podium placing for Canada in a Summer Games traditional team sport since 1936.
The Burnaby, B.C., native was a popular choice after scoring an Olympic-record six goals in the women's tournament, including a hat trick in Canada's 4-3 extra-time loss to the United States in the semifinals.
Canada went on to defeat France 1-0 in the bronze-medal match on a dramatic goal by Diana Matheson in the 92nd minute.
Canada finishes with 18 medals
The United States edged China in both the gold medal and total medal standings. The U.S. won 104 total to China's 87, and 46 gold to China's 38, recapturing the gold-medal title it lost four years ago. Britain finished third in golds with 29.
Canada finished with 18 medals. That's the same number the country won in Beijing, but the 2008 team has the edge in gold medals (3-1) and silvers (9-5).
Canada's one gold medal is the fewest the country has won since the 1976 Montreal Olympics, where it was shut out.
The 2012 team fell just short of the Canadian Olympic Committee's goal of finishing in the top 12 in the overall medal standings, ending up 13th in total medals.
"We have so much to celebrate as we bring these wonderful London 2012 Olympic Games to a close," said Canadian team chef de mission Mark Tewksbury. "Every medal has a story and you can not underestimate the power and the impact that each and every one has had. We are so proud to honour."