- Taoufik Makhloufi wins gold in the 1500m after being reinstated to the Games
- Algerian was initially disqualified from Olympics for not trying hard enough in 800m
- Australia's Sally Pearson takes gold in the 100m hurdles in an Olympic record time
- Russia's Ivan Ukhov wins men's high jump, Germany's Robert Harting the discus
Taoufik Makhloufi stormed to gold in the 1500-meters, making the most of his reinstatement after initially being thrown out of the Olympics for not trying hard enough.
The Algerian middle distance runner was expelled from the Games on Monday after the International Association of Athletics Federations (IAAF) said he didn't give his all in heats for the 800m.
Makhloufi had tried to withdraw from the heat but didn't meet the deadline. He completed just 200m of the race before pulling out, prompting the IAAF to act.
But after evidence submitted by a medical officer from the Games organizers, LOCOG, Makhloufi was reinstated and made the most of his reprieve by taking his first Olympic title.
American Leonel Manzano grabbed the silver medal while Morocco's Abdelati Iguider took bronze.
Sally Pearson asserted her dominance in the women's 100m hurdles by winning Australia's first track and field gold of the 2012 Games.
Pearson, the reigning world champion, set a new Olympic record time of 12.35 seconds, narrowly beating defending champion Dawn Harper of the United States while another American, Kellie Wells, claimed bronze.
"I don't even know how to describe it because it's like a dream right now," she told the host broadcaster. "I really wanted this tonight, I've worked so hard for the last two years.
"My goals were last year to win the world champs and this year to win the Olympic Games and I've done that and I couldn't be happier."
Robert Harting won the men's discus final with a throw of 68.18m to hand Germany their first athletics gold of the Games.
The 27-year-old kept up his formidable record in the sport, having not been beaten in two years, and celebrated by jumping some of the hurdles that were on the track during his lap of honor.
Ehsan Hadadi came second to give Iran their first ever Olympic medal in athletics while defending champion Gerd Kanter of Estonia took the bronze medal.
In the men's high jump, Russia's Ivan Ukhov took gold with a leap of 2.38 meters, edging America's Erik Kynard into second. Qatari Mutaz Essa Barshim, Derek Drouin of Canada and Great Britain's Robbie Grabarz shared the bronze.
Usain Bolt, who successfully retained his 100-meter title on Sunday, began the defense of his 200m title easing through his heat, shortly before fellow Jamaican Yohan Blake.
Team USA's Maurice Mitchell clinched the third heat, while France's Christophe Lemaitre clocked the fastest time of the morning to win his heat.
The women's 200-meter semifinals were also held ahead of the final Wednesday.
American sprinters Sanya Richards Ross, Allyson Felix and Carmelita Jeter will likely battle with Jamaica's Veronica Campbell-Brown and Shelly-Ann Fraser-Pryce for the medals after clocking fast times to qualify.