Defar pulled away from her tired rival to triumph eight years after taking gold in Athens, crossing the line with her arms in the air before kissing a religious picture she had carried in her top and falling to the track in tears.
Dibaba, who took double gold in Beijing and successfully defended her 10,000 title on the opening day of the athletics competition, had to settle for bronze.
A day after Kenyan David Rudisha’s blistering 800 metres world record, another full house at the Olympic stadium was treated to a world relay best as the United States, anchored by Carmelita Jeter, scorched to gold.
The American quartet of Tianna Madison, Allyson Felix, Bianca Knight and Jeter clocked 40.82 seconds to win the title back for the first time since 1996, smashing East Germany’s world record which had stood since 1985.
“It is a relief, it is a joy. it is everything,” Olympic 200 champion Felix told reporters.
“It is the most comfortable that I have seen this team. we were laughing, we were smiling…we have never been like that.”
The US, who failed to get a man into the individual 400 final, were denied a second relay triumph in quick succession when the Bahamas celebrated a thrilling first men’s track and field Olympic gold in the men’s 4×400 final.
Willed on by deafening roars, Ramon Miller ran a blistering last leg to run down Angelo Taylor inside the final 50 metres.
Frenchman Renaud Lavillenie soared over 5.97 metres, an Olympic record, to take pole vault victory while two golds were won by former dopers – Turkey’s Asli Cakir Alptekin in an eventful women’s 1,500 and Russia’s Tatyana Lysenko in the women’s hammer.
Dibaba, hoping to emulate Jamaican sprinter Usain Bolt and win the double-double, found her legs wanting in the home straight as Defar won in 15 minutes 4.25 seconds ahead of Kenyan Vivian Cheruiyot.
“I’m not very pleased today. I gave it a good shot but I wasn’t aiming for bronze,” said Dibaba, who could not repeat her blistering last lap of the 10,000.
“I’m a bit disappointed, but in a way I’m not sad because I did finish in a medal position.”
There was drama in the 1,500 metres where luckless American Morgan Uceny fell again in a championship final, having tumbled in last year’s world championships in Daegu, South Korea.
Uceny toppled over soon after the bell and, unable to comprehend her bad luck, beat her fists in frustration on the track where she remained, resting on her knees with shoulders hunched, until after the race had finished.
Not one of her rivals came over to console her.
Turkey took gold and silver with Alptekin, who served a two-year ban for doping after testing positive in 2004, edging out Gamze Bulut in a tactical race.
A day after proclaiming himself a “living legend”, Bolt earned a well-earned rest as Jamaica’s 4×100 team, the defending champions and world record holders, managed comfortably without the Olympic 100 and 200 gold medallist.
Yohan Blake, silver medallist behind Bolt in both sprints, ran the third leg and Kemar Bailey-Cole the anchor.
Bolt will return in place of Bailey-Cole for Saturday’s final as he seeks a third London gold and a sixth in back-to-back Games.
Justin Gatlin, bronze medallist in the 100, anchored the United States (37.38) to victory as they qualified one hundredth of a second quicker than the Jamaicans.
The US were shock casualties in the heats in 2008 when Darvis Patton, who ran the second leg on Friday, failed to get the baton to Tyson Gay in Beijing’s Bird’s Nest.
There was no fairytale medal for South Africa’s “Blade Runner” Oscar Pistorius as he said goodbye to his first Olympics before heading to the Paralympics.
South Africa, reinstated into the final on appeal, trailed in last of the eight finishers.
“This whole experience has been phenomenal, to step out here for an Olympic final is more than I could ever hope for,” said double-amputee Pistorius, who reached the semi-finals of the individual 400 and ran the anchor leg on Friday.
Australia’s defending champion Steve Hooker crashed out early in the pole vault final after three failed jumps.
Lavillenie added Olympic gold to his European title with Germany’s Bjorn Otto winning silver and compatriot Raphael Holzdeppe bronze.
Russia’s world champion Lysenko, who missed the 2008 Olympics while serving a two-year doping ban, won hammer gold with an Olympic record throw of 78.18.
Anita Wlodarczyk of Poland took silver with 77.60 and German world record-holder Betty Heidler claimed a controversial bronze with 77.12 after having her fifth throw remeasured following an appeal.
China’s Zhang Wenxiu, bronze medallist in Beijing, thought she had finished third again but was then relegated to fourth.