LONDON (Reuters) - Ethiopian Tirunesh Dibaba attempts a track double on Friday in the 5,000 metres to add to the 10,000 Olympic crown she retained in thrilling fashion a week ago in London, but two doping cases overshadowed another action-packed day at the Games.
Mariana Pajon, Colombia's flag bearer at the opening ceremony, earned her country their first gold medal of the London Games when she thundered to victory in the women's BMX event, where cyclists race each other over bumps and around banked corners.
Latvia's Maris Strombergs, a man who cried when he first saw a BMX race, remained the only men's Olympic champion in the discipline when he retained his title.
Tunisia's Oussama Mellouli won the men's swimming marathon though the Serpentine lake in London's Hyde Park to become the first swimmer to get medals in the pool and open water.
But there were fresh doping scandals involving a French and a Kenyan athlete.
France's 5,000 metres runner Hassan Hirt failed a test for the banned blood booster EPO (erythropoietin) prior to competing in London, where he finished 11th in his first-round heat on Wednesday and failed to qualify.
Kenyan athletics officials also confirmed on Friday that distance runner Mathew Kisorio had tested positive for a banned substance in June, but rejected his claim that doping was widespread in Kenya.
Earlier in the week Victor Conte, convicted owner of a now-defunct laboratory at the centre of a global steroid scandal, said it was easy to cheat at the Olympics by using drugs.
The International Olympic Committee dismissed his comments as being "like a poacher criticising a gamekeeper".
Spain beat Russia in their men's basketball semi-final and will meet either 2008 champions United States or 2004 Olympic winners Argentina in Sunday's gold medal game.
In handball, three-time men's silver medallists Sweden gave themselves a chance of a first Olympic gold medal by beating Hungary, and will play either holders France or twice winners Croatia in Sunday's gold medal game.
If Dibaba can she can win Friday's 5,000m, she will be emulating the 5,000/10,000 double of Finnish man Lasse Viren in 1972/76.
The Ethiopian retained her 10,000m crown a week ago with a scintillating last lap, demonstrating that she was back to her best after years of injury.
Jamaica's women will also try to keep their country's Olympic sprint magic flowing in Friday's 4x100 relay.
Women's double 100 champion Shelly-Ann Fraser-Pryce and third-placed Veronica Campbell-Brown will lock horns with a U.S. quartet featuring 100 silver medallist Carmelita Jeter and 200 winner Allyson Felix.
The men's 4x400 final looks like an open race, with the Jamaicans having failed to qualify and the United States, seeking an eighth successive win in the event, not as strong as usual.
Friday also sees medals on offer in the women's 1,500 metres, women's hammer and men's pole vault, while the heats of the men's 4x100 relay should give another sellout crowd of around 80,000 plenty of excitement.
The United States stand atop the overall medals table with 39 golds to China's 37.
The two have been neck-and-neck throughout London 2012 in the race for Olympic bragging rights, but whoever wins, home nation Britain will certainly be celebrating their best medals haul since 1908 when London first hosted the Games.
Their 25 golds put them behind China in third place and easily surpassed their 2008 total of 19.
The golden glow has helped fuel the popularity of the Games among a public that has packed many venues and lifted athletes with deafening cheers.
IOC President Jacques Rogge gave the London Games a lavish endorsement, telling London's Evening Standard newspaper: "The superlatives created here in London will live on long after the cauldron is finally extinguished.
"In the true spirit of Britain, huge crowds have cheered on not just their athletes but those of the world, and sent an echo that resonates in every home across the globe," he said.
Travel delays have not been as severe as some predicted despite a surge in travellers. Transport for London said that in the first 12 days of the Games, 47 million journeys were made on the Underground system, up 30 percent on a year ago.
There were disappointments for home fans on Friday though, including on the water where Australia won gold ahead of Britain in the men's two-hander 470 sailing class. New Zealand repeated the trick in the women's event.
Tunisia's Mellouli thought he had reached the peak of his career when he upset Australia's Grant Hackett to win the 1,500m freestyle gold at the 2008 Beijing Games and seemed almost surprised to have added the open water swimming title.
"After winning this gold I will definitely think about retiring because I don't think I can top this achievement," he said.
Attention turned to Sunday's closing ceremony, when the Olympic flame will be extinguished.
Before the baton is passed to Rio de Janeiro, which hosts the first South American Olympics in 2016, the world will witness a ceremony celebrating British music, with the Spice Girls, The Who and Brian May among those set to perform.
(Reporting by the Reuters Olympic team; Editing by Michael Holden)
Related Stories: Factbox: Memorable quotes from the London Olympics